Also, it is possible to continue iteratively improving a complex piece of software like a browser, beyond just the web standards race. Security, privacy, performance, and reliability/bugs/code correctness are areas where new computer science is constantly coming down the pipe and worth integrating. And maybe other things like AI or features for AR/VR systems, though it's more debatable whether those belong in general purpose browsers.

Browsers aren't Unix utilities that should do one thing and just one thing and thus can theoretically reach a state of "done". But even there it's not always a certainty. For example, "sudo" being superseded by a simpler more secure "doas", and then more recently by SystemD "run0". Even simple utilities continue to evolve.