Well, that's easy: Bad choices. The better something is, the easier it is to change it in a way, that makes it worse.
I agree with GP, everything can be improved, but that doesn't mean that you will succeed, or that you should try, for various, separate reasons.
The point at which new changes are more likely to degrade an application that's already excellent for its established use cases than they are to improve it further is the point at which we should consider the software "finished".
There may be lots of theoretically plausible improvements that could be made, but if the constraints of actual practice make pursuing those ideas a net detriment, that's probably a good place to stop.
I think AI may actually make this simultaneously better and worse: it will eventually be really easy to hit all of the functional and performance targets early on, so "finished" software will at some point be much better and closer to true completion, but the mentality that drives people to keep fiddling with it instead of moving on to a new project will be just as prevalent, and the post-"finished" releases will likely be even worse.