I respectfully disagree. I see Indy heading in the direction of the Bond/Batman franchises. In that scenario, every 3 or 4 movies we see a new Indy actor/director injecting new life into the character and franchise. Which is why I see moving on from Spielberg as a necessity. Sadly, its probably time to move on from Harrison Ford, too.
On a side note: I remember reading that Spielberg had little interest in any sort of Nazi plot following Schindler's List. Maybe LucasFilm is demanding that classic formula for this film.
Wether it is nazi's or not that is the most likely explanation. What Spielberg wanted to do with the next Indy was likely different then what disney wanted done and so be decided it was better to leave then not do what he wanted to do.
For the most part, I'm of the opinion I'd rather not see somebody else take over Indy after ford however I'll contradict myself and say I did enjoy young Indiana jones chronicles (except for the real young kid)