As much as it's true that a novice will generally use AI to build a sloppy mess, I've also had success unsloppifying through some careful prompting.
If I had AI tooling at the time I'd probably be more inclined to have it both refactor / optimize the existing application, add automated regression tests etc, and use it to extract all of the features and requirements for it for a potential rebuild.
But honestly I think if that application was properly designed and factored (instead of nesting JS in HTML in strings in JS or concatenating XML from query results only for it to be converted to JSON taking up 50% of response time) its lifetime could've been extended, especially if it was then containerized into a HHVM or similar php optimizer.
But, hindsight.