How is it probable that it'll be better than KINGDOM?
Well, frankly, I think Spielberg will have to try pretty hard to make a film that's worse, and it's honestly fairly easy for him to make a film that's better, if only marginally so.
I also suspect that Spielberg and co. will benefit from having learned "learned some lessons," so to speak. After all, much of KINGDOM was made on the fly, without too much pre-production, and things suffered as a result. If you read interviews, it's pretty clear that the final film isn't something Spielberg and Lucas were enamored with (in fact, the difference between their pre-production comments and their post-production comments are pretty striking).
There's also the matter that KINGDOM was hampered by constant bickering over the core narrative concept. In the end, Spielberg put up with it, but he still didn't like it. This time, it seems that Spielberg is actually fond of the concept--in fact, he came up with it himself--and the other two are on board.
No offense, but Ford actually believed the script for Crystal Skull was good enough to sign onto that film.
It's hard to say what the script looked like when Ford actually committed to the film, since the final product we now have was something that only really settled into its finished form late into pre-production (and even during production, since there were near-daily rewrites).