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.