Websites are either made to only be as good as what the worst browser can render or needs enough developer time to work around it. In other words as if speed limits were set to 10mph on highways because my crap SUV dictate the road for everyone around me. The argument gets turned on its head when using a car analogy though. If only Google-sized companies can build a modern rendering engine then they dictate the web (and they rarely agree with users on what's important). If Ford could dictate roads they would make them impossible to drive on in other cars - or if Google did they would mandate only cars with RoadAd™ turned on, if Apple did only cars with iTires™ and so on. It's the road that gets dictated by the big companies like Apple and Google, not the cars. Roads luckily work just fine in an old or low-production car, unlike the web.
The argument that Google basically decides what a browser looks like is interesting. The negatives are often theoretical, "what if they just built ads into the engine" or privacy-oriented "they slurp your data". I pretty strongly believe that Google (and perhaps Apple in some respects) pushed the web far beyond where it could've stagnated, probably for self-oriented purposes. If you want to see a browser without selfish advancements, compare IE11 to its contemporaries, on their release and then after a few years. Microsoft had no reason to force new technology into their browser experience, so they didn't, and their browser turned into a boring antique.
You mention that "[Google] rarely agree with users on what's important", but I'm not sure that's true. I think the HN crowd, and those adjacent to it, often disagree with Google's practices, but my intuition is that Google's browser changes have been effectively silent improvements for the vast majority of users. If you installed a massively outdated version of Chrome on a random user's phone or PC, they'd probably be almost immediately upset at some missing feature or function.
At some point, these selfish advancements either become obsolete via the "official" way of doing things being implemented, or become themselves part of the spec. Should Google, Apple et al. have this much control over the future of the browser spec? My instinct is "of course not", but when I think about the history of the web, and how important these selfish decisions have been, it becomes harder to decide.
My original point you're replying to was moreso "why should a small independant team be able to build a browser", and I'm not sure you replied to that.
They are not theoretical. Too bad webapicontroversy.com has been shut down (it looked like this [1]), but you can scroll down to "defer" and "considered harmful" in Mozilla's positions here: [2]
There are more, of course, but they are not visible unless you're willing to follow thousands of issues across hundreds of GitHub repositories. One that springs to mind is, of course Constructible Stylesheets. Mozilla and Safari: the spec describes an algorithm that leads to deadlock in trivial code, we wont implement it until this is fixed. [3] Chrome: ship it, because lit-html (developed by Google) wants it and is already using it. And then procedes to gaslight people and misrepresent their positions (cant' find the relevant link, but at this point I can't find the will to dive into the cesspool).
Forcing changes to the spec/implementing non-spec features is one of many possible negatives of browser oligarchy, though. When I said the negatives are often theoretical, I was referring to the other arguments that are commonly raised, surrounding pernicious updates and privacy concerns.
