Everything else I said, sorry if that wasn’t clear!
Essentially there are organizations like W3C and IEEE that define standards for how the internet works and how websites behave. All browsers follow these so everything works properly. Let’s say you have some idea you want to add to your browser you develop. You do it and tell everyone about it. You don’t have many users. Maybe a few sites do it but it isn’t really a problem that it doesn’t work on other browsers because so few people do it.
Chromium has a massive market share because so many browsers use it as their base. Even Opera and Microsoft Edge which historically have been alternatives to Google Chrome now use Chromium as their base. The danger is that Chromium has such a large user base that they are essentially what the standard is.
As a quick aside, Chromium is the name for the open source base of Google Chrome. Chrome itself is technically not open source. This jus thust in case you or other readers haven’t seen that word.
Imagine a world where everyone uses Chromium. Why would you (if you were in charge of Chromium) need to listen to what standards organizations say about how the web should work? You’re literally in charge of every browser! You can just add some new features or take some out and every website would have to comply because you (in this hypothetical) truly do control every single web browser on the planet. Their websites would not work otherwise.
Sure, out of the goodness of your heart you might behave and be a good steward but there will always be reasons for you to act against the standards that you don’t view as “bad” that other people might think are bad. I’m not saying all standards organizations are perfect and good or anything like that, but I believe I trust them more than Google.
Even if Google never does anything “bad” (naive thinking lol) avoiding the situation where they have that kind of power is a good thing.
To me that’s the most important reason to use a non-Chromium based browser. To avoid Chromium becoming the one true browser.
And just for some context, Google has done bad things before with regards to web standards and then having the de facto standard with Chrome. The recent changes to the extension API to neuter ad blocking being a prime example. And we don’t even have to speculate and sound like nutjobs. They’re a public company. They’ve said before that ad-blocking is one of the biggest threats to their ad revenue. Not that it feels tin foil hatty to suggest even if they hadn’t said it, but they actually have said it in reports.