I still don't get why people were so down on Crystal Skull. Even as a kid, I thought the Temple of Doom was kinda dumb, but still enjoyable, and I'd say it's definitely the worst of the series. Also, I thought the official reason they definitely didn't want any Nazis in a new film was because Spielberg was still traumatised after making Schindler's List.
In any case, I can sympathise with the whole "recasting for a new generation" thing and the Bond precedent but I feel it's missing a point both narratively and socially. Indiana Jones isn't 007 - he's not a cog in the machine, he doesn't fall into the pulpy spy world of disguise. Recasting him doesn't add a layer of understanding to the Indyverse as recasting Bond does to his universe. All it does is draw attention to the fact that it's a film series, a story, and your immersion is fractured every time a comparison is made. And thanks to the internet and the unbelievably weird Indy fanboys, it will be absolutely impossible for a younger generation to view this as "Indiana Jones." It'll have to be "their Indiana Jones" at best because no matter how good the film is, as soon as they try to connect and discuss via the internet, some jack off is going to start shouting them down about how "Raiders of the Lost Ark was the first one (which came out before you were born) and is therefore THE ONLY POSSIBLE GOOD ONE AND YOU'RE DUMB FOR LIKING YOURS HURRDURR." In fact, I think someone has already said words to that effect in this very thread. It'll be so difficult to have that sense of agency that endeared Jones to young kids because they'll be always reminded, never mind how they feel, that it isn't as fresh as it was before. Which is really quite sad.
My dream would be for them to put the franchise to rest. It's done, it's fine. Indiana Jones is a film franchise, not a serialised comic book, it's allowed to end and have closure in its story. Maybe it didn't end on the high note you wished for but hey, that's goddamn life and I would have thought that over 10 years of fallout from the Star Wars films would have taught you that. Make something new, for the new generation. My personal choice would be to bring back the swashbuckler genre; with the benefit of modern social progression, globalisation and, of course, CGI, that's a genre that could seriously explode into a huge cloud of money.