User-facing software in general tends to hit a quality ceiling pretty early on based on the unwillingness of the developers to make further improvements. Contrary to the commenter who talked about NASA and the space shuttle software, this is actually a matter of willingness, not ability. I've filed half a dozen reports on bugs that would have been trivial to fix, and been refused every time. I've seen developers go out of their way to reduce the quality of their software and break things that used to work.

On the bright side, this means most domains of user-facing software still have open opportunities even when they should be thoroughly mature. If only there were more hours in a human lifespan, I'd love to fork Chromium or Firefox and just start fixing things at the browser end.