Exactly this. Having Firefox as a major player is necessary to essentially force web developers to consider Firefox users in optimising websites. Having a 100% Chromium web “ecosystem” so-to-speak would effectively render Google as the king-maker, and with that we can essentially say that privacy is ruined.
Microsoft is currently the only 3rd party capable of maintaining their browser even if they lost access to Chromium code. They have patched certain vulnerabilities early on, so they aren’t necessarily in danger. With other browsers, there’s a lot of possible risks. Chromium could just be removed from the market, or even locked behind a paywall. In either case, only Microsoft has a chance of evading possible issues.