I think the recent trend has to do with the age of the people involved more than things changing.
Arnie for example got very old, he's not built like he was in his prime anymore, and that kind of shows. You can't have him tear off his shirt for the climactic hand to hand battle with the main villain and go "OMG", and really Arnie being able to carry off the whole "I'm superhuman, but I'm real" was kind of his appeal. He's a decent actor, and pretty smart, but don't forget he was also a living prop. It's also notable that Arnie became famous not just by playing an over the top action hero, but doing it in some rather creative science fiction settings. It's been long argued that James Cameron was as responsible for Arnie's success, as he was himself. If you look at Arnie's straight action movies like say "Raw Deal" you'll notice they haven't exactly been among his best. The big movies that made him famous were as unreal as he was, "Total Recall", "Predator", "The Terminator", and others. His most recent movie bears more resemblance to his relative failures than his big successes.
In Arnie's case you have to also remember that information technology has grown since his heyday. Arnie always had a reputation for being something of a macho jerk IRL, there were articles about him in connection to movies like "Pumping Iron" that are negative, with titles like "Arnold The Barbarian", but there was really only so much of an audience this kind of thing could reach and bombard you with, his exploding popularity made the people singing his praises a lot louder than his critics. Arnie comes back from a relatively divisive political career, and gets in trouble for banging his maid under the table... everyone hears about all of this stuff, and I think that very much influanced his performance in a way it never could before.
With Stallone it's a case where he's also still built, but again is showing his age, and they have largely had to gloss over his decreasing physical abillities. When the appeal, like Arnie, is that your action hero looks like he could really do the stuff the character does, combined with decent choreography, it loses a lot when your living prop no longer fits the bill.
Something like "The Expendables" succeeds because while the props involved are aging, you get to see them all together, and they share the burden of carrying the movie. The star power also lets you forget how terrible a lot of the choreography is and the tricks they use at specific moments so you don't ever clearly identify the actor doing something, since they are using more stunt doubles. There were always stuntmen involved, but I think it showed a lot more here than it did before, and it's something people realize is going to be a problem when an aging Sly or Arnie tries to carry a movie entirely on their own, your going to be paying mostly to see someone pretending to be Arnie doing an action movie... and people catch onto that kind of thing.
With "The Expendables" it's also noteworthy that along with the musclemen who knew some decent choreography, you also had some pretty skilled martial artists. Jet Li, Jason Statham, and Dolph Lungren (believe it or not) all have pretty substantial martial arts backrounds, there is a reason why they had Dolph fight Jet Li for one of the big scenes in the first movie, and it was also one of the fights where you got to see the most of the actors, being sure it was actually them doing the stuff.
To be honest there is still a market for this, kind of thing, but a new generation of action guys never really appeared, and it's easier to use computers than it is to actually find a bodybuilder who can act. Likewise, one also has to remember that the most successful movies of the genere tend to have some wierd spin to them, the movies where a muscleman just beats up a bunch of derivitive criminals with poorly written justifications have generally not been the ones that made stars out of the action heroes, generally forming the filler in these guy's careers.
For a while I thought Vin Diesel might actually be the second coming for the genere, but he generally wound up making some terrible desicians on what movies to do.
I also think political correctness affects it as well. I know I say it all the time, but the bottom line is that when you let liberals run rampant, you wind up with a situation where nobody can be the bad guy anymore. This leads to really having to stretch things to try and make it so the bad guys don't offend anybody. When people actually get press for saying movies like the last, terrible, crysal skull based "Indiana Jones" movie were bigoted for making the bad guys Russian, you know there is a problem. You have people fight criminals nowadays people will scream it's insensitive to whatever culture is the target no matter how close to the truth it might be ranging from blacks (gangstas) to Italians (the mafia). When all you have to do is drop an "ist" or an "ism" to get a platform and five minutes of fame, people will look for any excuse to do it. This means that by definition any kind of "reality grounded" action movie has real problems since it has to stretch what is already a fairly weak premise even further... like it or not, you can't have an action movie without bad guys to get beat on, and those bad guys have to be a group the audience believes represent a credible threat.
Let's say today I decided to make an "old school" Arnie epic, meaning I'm going to use a combination of reality and crazy fantasy stuff. I cast our old geezer of a hero as a retired general having come back from decades of service to his old home in Texas, which happens to be terrorized by Mexican drug smugglers and illegal immigrants, a group which is not only untouchable because of their numbers and guns, but because they are a bunch of Santa Muete cultists (the saint of killers) based loosely on the real deal, with the skeletan emblems with the gaudy jewelery on them and everything. The leadership having magical powers, and there being an actual manifestation of "Saint Death" running around doing the bidding of the cult leader. As a result you have Arnie come out, beat up a bunch of gang members, make a political statement (kind of contrary to some of what he stood for in office) by loading a bunch of immigrants into the back of the van and dumping them over the border and threatening to shoot them if they come back, intoning "problem solved" or other typical deadpan one liner as he does, before eventually getting the irritation of the cult's leadership who send some guys with super-powers after him. Arnie sends off a message to some buddies that work in a goverment lab (since he was a general and all) and goes to war with them, doing pretty well despite their powers (I mean he is arnie) but ultimatly being on the losing end. After hiding to recover from injuries a shipment of super weapons arrives for the general and he heads out and unleashes science fiction firepower on the rampaging cultists. The leader however gets upset at this point, channels his god, and you have the big showdown between it and arnie, with laser beams flying all over the place, and a finale invoving bone crushing hand to hand combat which arnie loses, before lowering it into an ambush near the border (turns out he called some military friends who also have super weapons, he was just holding it off for them to arrive) before decimating it, throwing the carcass over the mexican border in front of a bunch of watching cultists and the illegals he tossed out before, before intoning "consider yourself deported" end credits role.
In short epic, politically irreverant, cheeze. The exact kind of movie guys like Arnie are supposed to make, and which tend to be utter bombs, or explode into popularity where the sheer stupidity of the thing winds up making it even more epic. Half the point of such movies are also to give the mainstream seemingly simple solutions to problems that are more complicated in reality by way of escapism. Nobody is going to solve problems like illegal immigration or the border drug wars with a bit of two fisted head busting, but it's nice to put your brain into neutral for a while and pretend. You can then laugh about (and think highly of the movie) when things get too deep on an issue you can defuse it by saying "Consider yourself deported" with an Austrian accent for a laugh.