Imagine you reach peak usability in $YourApp, in the year of our lord 2023. It's maybe not perfect, but there's no conceivable way your app's usability can improved past this point. Users are reasonable proficient, everything mostly clicks into place past the inherent hurdles of learning to use an app.
Can this situation stand? Well, no. If you keep your UI as is, the software world will often perceive your app to be "stale" or even dead. So you have to change. But if you change anything after you reached peak usability, you will necessarily worsen your UX, sometimes in catastrophic ways.
I cannot prove it, but I think this is one of the reasons (not the only one) that explains why some app's (and desktop environments!) decent UIs get ruined so thoroughly.