I agree, no one should ever bother writing a new browser again, too risky. We should just use chrome forever
Also doing that doesn't solve the standards problem we currently have.
Google is able to push through any "standard". And those standards "unwittingly" help them maintain their search/ad dominance or prevent competitors. For example see manifest v3 or FloC (a.k.a Topics API).
Once they gain enough traction and become indispensible, you either implement them or risk losing users.
https://gs.statcounter.com/browser-market-share
which means Google has significant control over web standards
There will be two classes of products one officially sanctioned version and the others that are used by enthusiasts. Apps or sites in this case may chose to work on one and not on others. Imagine the new wave of "works best on IE" with "best viewed on chrome variations a,b,c". Its not farfetched as some sites already do this.
As much as it is easy to maintain a fork, it is that much easy to give up or change path and accept upstream changes.