It'll go well for a while (and users even might love you for it for a year or two, since you can focus your effort on fixing bugs and not introduce new ones with new features), and then you'll slowly but steadily lose users to other browser that do adapt.
Losing users for a web browser, means losing search referral revenue in the short term (literally Mozilla's lifeblood), and losing web developers in the long term, which will break the experience even further.
Just one example that almost made me switch browsers: Web site translation. I was regularly using Chrome in parallel for that, but now Firefox fortunately supports it too (and in a privacy-preserving local way at that – a true innovation), so they keep me as a user.
