The main benefit to JS being used as a server side rendered and client side rendered language is that you have a lot more control over which parts you want on the server vs the client, and you can introduce hydration where needed. You technically could do that in PHP and other non JS languages but that's annoying because you'll be mixing multiple languages together and it would be more clunky. So yes, there is some "revolutionary" benefit to having everything in JS/TypeScript.

I'm not really in webdev, although I still do django sites for people sometimes, amazed and glad I had no idea that's how things had gone.

Aren't those just the server-side components from React?