I mean, I made it to the 90-95% completion mark by the end of the jam and already put a good bit of time afterwards into working on polish and bugfixes - and I’m pretty sure a number of other creators are in the same boat. Ideally, I’d like to guide players towards a more playable build directly through itch than to update the game profile or respond to bug report comments by saying “this is bugged, I’ve already fixed it, but you have to download it from [Site X] instead”.
On the other hand, I can also understand how allowing newer builds to be uploaded could influence the community ratings, and even with a requirement to leave the original build intact, it’s not difficult to obfuscate what was or wasn’t available in the jam version.
In all, though, I’m in favor of allowing submitters to upload new builds, though perhaps with stipulations that the original game jam versions be clearly marked as such, and that the community shouldn’t leave ratings based on the post-jam builds of the games before the judging period ends.