I don't think they can ever get rid of actors, or rather they can for the cheap blockbusters, but not for higher class films. Because even if the graphics become completely cheap and realistic, the trick is in doing the actual acting, programming every little gesture and movement. And I just don't think it will ever be cheaper for someone to manipulate model to get that information, than just paying someone to do it. I mean if you look at all the game cutscenes today, they're almost universally motion-captured to some extent because it's easier to have a human move about than try to animate to make it look perfectly like a human moving about
The actual appearance of an actor will being to matter less though as it becomes more mainstream to photoshop someones appearance in a film and generally mess around with the looks as they see fit. This might mean that we lose the big names of actors, because mo-capping onto a different appearance might create a different sense of identity, but if acting is actually difficult and there's a limit to the number of great actors in the world, then people are still going to want to see the ones with the good actors doing the motion capture.
Another thing is, I imagine it will be very hard to create a completely different look that looks realistic and again, it's probably cheaper and easier just to scan a real person in. But it might mean that we have models 'sell' their appearance, which then gets touched up, and have someone else do the acting for it.
Maybe when computers get so advanced they can more accurately gauge and piece together human gestures and emotion than we can it will change. But by then we'll have already faced the serious question of how life will function when machines are actually smarter than us, and films are the least important consequence to worry about =D
In the shorter term, I don't see anything changing. Films survived TV, so I don't see the internet as too much of a threat to them. Maybe cinemas might get replaced by streams, but people like meeting up with friends and physically going places. I wouldn't be surprised if we do start seeing some films get streamed simultaneously with a release though. At least when people feel like the technology is safe enough that the pirating won't lose them money. (In all honesty, this is something companies could start doing now (are there examples of this? Sky Box office stuff i guess?) but I think they're a little afraid of technology and also they'd probably be cutting down their potential revenue a lot because all the money from popcorn sales and the like would be cut from the industry chain. Also people are less willing to pay on the internet.)
And so I can't see anything thats going to disturb the blockbuster cycle ever. Films will probably get even quicker paced now though. Already the way people's memories work are changing, because it's becoming pointless retaining information, when remembering how to acquire it is much more important. (I mean why should I remember x-date from history, when I can find it in less than a minute from pretty much wherever I am in almost any conceivable situation where I would use it. It's just using up brain space). I'm 20 and I didn't completely grow up with the internet, and smart phones hit towards the end of adolesence, so even I'm not part of the true information generation. In 6-10 years time the true smart-phone/internet generation will be hitting and the way their brains process information is going to be even more quicker and demanding. (My younger family members routinely split a computer screen and perform multiple tasks whilst a video is running as well). Movies will reflect this. People now might complain Michael Bay is messy and hyperactive but (apart from the poor sense of geography) that stuff's going to be clear as crystal to young people in the 2020's and the equivalent of Michael Bay then is going to be more like Moulin Rouge