The only issue I see is that people don't want something else entirely, they want their web pages. CSS/HTML features are deprecated but browsers continue to sup

The only issue I see is that people don't want something else entirely, they want their web pages. CSS/HTML features are deprecated but browsers continue to support them forever and make their engines more complex because... people won't adapt (for good and bad reasons). What you're proposing is basically telling someone willing to build a better web browser to compete with Chrome that they should actually build, say, a music player instead and change the game completely. Even a less extreme example: a browser with a completely different UI still would need to support HTML/CSS and have a super complex engine so we're back to square one. I don't see how that makes sense. We need more energy and money to be spent into moving this distributed codebase to use newer features so engines can drop support for old stuff and be less complex. Who's going to drop features from their browser only to see people say "use browser X, that one still works with all your existing pages" and then lose their userbase. If this was easy as ever, as you say, people would have done it already. In reality, it's a multi-billion dollar problem and the incentives don't align.