I disagree. The Adventures of Tin Tin proved he could still make an purely fun adventure movie if he wanted. In many ways, that was the peak of what the Indy movies could be, or rather, the "idea" of them. I certainly don't think it's a great movie with its choices but it was fun and the most fun I had with a movie until Fury Road was released. Crystal Skull, Spielberg has lamented about since and during and there are interviews with him then and now that pretty much says he did it because he likes the Raiders movies, and George, and thought it'd be fun getting the "Indy gang" back together. Fat lot of good that did.
As far as I know about Spielberg leaving the project early on, it was over "creative differences" which to me means that he was told to do something with Indy that was antithetical to his character (again, something Spielberg had direct influence over) and that it was an unshakeable term, he'd rather not be involved at all than do it. Mangold stepping in to helm it seemed like a "make lemonade" situation. The project was going to happen in spite of Spielberg and here's someone who'll play ball and also do his best to make something decent with what he's given. What that "something" will be is what's got all of us worried about.
