> But if the vendors really would just sort of fix their old mobile browser, you would still be stuck with a old browser unable to interact with the modern web.

This is a non-problem nowadays. We have long left the times in which browsers received essential features every few weeks. Using a browser with a feature set from five years ago you can still use all the most-visited websites perfectly fine. At the worst you're unable to use small, non-essential features of some sites. Maybe some ads look less fancy ;-)

Your problem today as a browser user is security against zero-interaction exploits, not missing out on some obscure brand-new CSS features. Security updates are thus what you need first and foremost.