Here's my question: in light of what Mozilla is doing, why don't other forks like Waterfox or Librewolf write a manifesto/contract saying they'll never sell your user data and won't turn "evil" (until they do, of course), and then decide to offer a paid version of their browser.

Two possible outcomes:

1. No one cares. No one pays for it. Nothing changes and nobody loses anything.

2. Enough people pay for it to keep the product healthy and the user-centric promise alive. The Internet is saved.

So why isn't anyone trying to replace Mozilla yet, with a more sane business model than living on the back of Google's fear of antitrust investigation? What's the worse that can happen?

Just sell a bonafide paid version alongside the free one, don't just rely on donations. There is a massive difference between offering a paid product and begging passers-by to spare some change.