The problem is the lack of diversity. Google controls Chromium and almost all browsers are Chromium based so Google controlls the supported web features of almost all browsers, giving them the power to decide which web features are supported on the internet and which aren't. They use this for example to push their own file formats for the web instead of better alternatlives. Remember when everyone was mad that ublock origin wouldn't work on Chromium browsers anymore? Same thing. They get money from ads so they make it harder to block them. Google shouldn't have that much power over the web.

One big life-changing thing will be something like Manifest V3, limiting ad-blockers capabilities. https://www.eff.org/deeplinks/2021/12/chrome-users-beware-manifest-v3-deceitful-and-threatening https://www.eff.org/deeplinks/2021/12/googles-manifest-v3-still-hurts-privacy-security-innovation

Rules for thee and not for me kind of stance when it comes to who is collecting all the sweet data from its users, it wont be possible to block every tracker and ad from Google in the future because every add-on will have a limit of how many domains they are trying to block.

I mean what could possibly go wrong if the biggest data collector and ad provider has a monopoly on web browser?

Why is it bad to use chromium base browsers? If the market is essentially only Chromium and Safari, Firefox compatibility will be even less important and broken sites will only lead to a bigger monopoly because users will switch.

Mostly having the centralized backbone of chromium makes people uncomfortable since it can do a lot from behind the scenes if it wanted to. But the raw base form is pretty much fine AFAIK, it's just very rare among browsers to do so.