But that's just the perspective of someone firmly wedded to the backend.
Angular is a total rewrite of AngularJS, that as far as I know is incompatible with the original (no firsthand experience, going off what I remember others saying about it). It was first released ~6 years ago, while AngularJS's last release was ~2 years ago.
No comments on Vue, never used it and I don't hear about it as much as the others.
About the other two: like you said, both React and Vue can still run old code, almost unmodified. The track record of those two is way better than most backend frameworks, by miles. It’s not even a fair fight.
Sure there's new concepts, but it's not arbitrary stuff or old patterns, it's legitimate stuff that makes writing UIs better. But the old ways are still there too.
Backend work today seems less like "knowing a bit of SQL and PHP" and more like "knowing a bit about these 5 competing new programming languages".
I find it frustrating when my clients employees actively refuse to dive deeper because they "don't know how/I'm not experienced enough", well, then it's your job to figure it out, that's why they pay you money.
As long as most of us are "just" implementing bog standard web frontends and bog standard backends using some sort of SQL, then I find there's no excuse for not being able to pick up the slack, if you're asked to.