Think about it like in this oversimplified/stupid example: A "fix" or change is implemented in Chromium. In the long run (intended or otherwise) it turns out it moves pixels slightly differently than the way Firefox does it. If almost all your visitors use Chrome you have to design your site to be perfect in chrome and you might do so in Firefox. Now you have sites that look as intended in Chrome but maybe look as it should in Firefox. This make Firefox users use Chrome more.
Now Brave et. al. is part of the problem, helping drive the only real competition out of market (and killing their only way out should they some day need to change engine).
In extremely complex code this is very hard to not be a part of and a company like Brave is way too small to fork Chromium for long if at all.
What this means is that this engine becomes the de facto standard of the web and this standard is controlled by the main contributor of the engine.
Every browser is now constrained by Google's own decision about what should the web be. Sure, they could technically disagree by forking WebKit/Blink, but since websites are made to work with Blink, a disagreement means being incompatible with such websites.
That's already the way it works in the real world... the standards are irrelevant and ignored, only caniuse and browserslist actually matter. Like it or not, Blink is the new IE6, and its marketshare is only increasing.
Ideally it would be something not controlled by Google but by an independent third party (hand Blink over to Mozilla, deprecate Gecko?), but good luck with that.
Maybe this system wouldn't be as ideologically pure as building compatible renderers to a set standard, but it would result in far better developer and end-user experiences as the web quickly standardizes to a single renderer. The world simply does not need 10 different ways to display HTML with 90% compatibility.
Of course those are just random made up ideas but the point I want to make is that it's giving only one actor the power to define what the future of our only and sole international knowledge network will be.
One way or another, browsers are heading towards engine homogeny (or hegemony), but Firefox and Safari are at least slowing this process down to some extent.