The older I get, the less I want to use new browser features that have already been somehow possible with existing ones that typically have been around for literal decades.
Adopting them makes a future with multiple browser engines more labor intensive and unlikely as the number of standards targets one is required to support grows and it becomes a game of asking yourself what sites you want your browser to support.
If you don't have a lot of experience in web software, you might think, that's a silly thing to think about, but I think it's that level of maybe exceptional thoughtfulness that has really profound ramifications. If you do have a lot of experience in web software and you think it's silly to think about, you suck.
You started that off by raising an interesting point. Pity you had to ruin it by telling us you're exceptionally thoughtful and profound, and that those who disagree, suck!
It's true, the amount of work that would be needed nowadays has made the idea of building a new browser engine almost impossible (I don't know how the Ladybird devs are getting on). It's a reason to encourage people to support Firefox on desktop, since just about everything else is based on the other browser engine. I still miss Opera's Presto - we don't want to lose another one, or there'll be no competition at all.
But as we've already crossed that threshold where the workload is preventing new engines from being built: choosing not to use new features, after they've been added to all the existing browser engines, isn't going to help that situation in any way.