Compatibility is important, even for the market leader, because if they break too many websites too quickly, that will push people to switch browsers like nothing else. Especially if it's a big website people use every day.
Chrome does have a somewhat easier time taking the lead on deprecating things but it often requires multi-year campaigns and gradual steps. (Consider the campaigns to kill NSAPI and Flash.) This is needed even for Google to maintain compatibility with its own websites.
The analogy to markets still works. Ebay can change the rules to be more buyer-friendly but not so much that too many sellers leave, because buyer-friendly rules don't matter if you can't find the thing you want to buy.
An example of the market breaking down is major news sites blocking Chrome's Incognito mode, despite Chrome's gradual attempts to make fingerprinting harder.
(yeah, I know they have to care about website designers, otherwise every website will just break, but when you have substantially a huge share of the browser share, you can tell website designers to get stuffed and they will have to deal with it)
I'm being intentionally simplistic- sometimes, complicating things with markets and so on feels like it obscures more than it illuminates.