I personally beleive they should've made a film 3 years after ToD... And I don't believe this nonsense that "they couldn't come up with a good story" for 20 some years until just a few years ago when they started the process for KotCS... Look, just a few months ago I started on a storyline for Indy 5 and I've already come up with a whole boat loads of ideas. Ideas are not hard to come up with. I just believe they were just tired of making them, and it was just too taxing mentally to come up with another one... I've run into the same problem when I create scripts. A lot of times, after a while, you just don't feel like it anymore.
I firmly believe he should've done Kingdom of the Crystal Skull 3 years after The Last Crusade (maybe even 4 or 5)... then come back and do Indy 5... then Indy 6... then stop for a few years and have River Phoenix do some movies as Young Indy (movies, not TV Show)... but sadly, Phoenix died from stupidity, so there you go...