While other browsers technically exist, it is foolish to think that web browsers are a thriving diverse ecosystem right now, when 74% of all web-browsing is done using a Chrome-based browser. With their influence, if Google decided to start forcing changes on how websites function on a technical level, they could absolutely do that with little to stop it;- what are websites going to do, alienate a supermajority of their users?- And that is why people are so worried about things like Threads, because once a single company has supermajority control of a market, they can use that control as a weapon to get what they want.
I say this as an avid Firefox user: Firefox is niche. And the only reason Safari has 20% is because it is integrated with apple products, if it weren't for that, Chrome and Chrome-reskins would effectively be the only option.