Working without javascript requires development time. Running old versions of HTTP doesn't. Version 1.1 is going to be fully supported for a long time.
Running old versions of HTTP requires nothing because people will need to support legacy devices for a long time, and it's obviously already implemented in all major components (Servers, CDNs, Clients, etc.)
You (will) run HTTP/3, because it is more efficient, meaning that your servers will be able to service more request.
You run HTTP/2 and HTTP/1, because a lot of people are still using that, and you don't want to lose them. This especially applies to mobile devices, many of which are stuck with software that cannot be updated for various reasons.
There's no threat of the majority of websites going HTTP/3 anytime soon. By the time that might be a possibility, Tor will catch up.
Unless they lose 99% of their users, there's enough demand. Your level of pessimism on this specific detail is ridiculous. Tor might not last forever, but it won't be lack of HTTP support that kills it.
If you wait a few years for HTTP/3 to settle, proxies will be available that could be glued into tor inside a weekend hackathon.
Still not getting it. Every site is hosted. The hosting company needs to spend money on HTTP/2, and be allowed to use it. The networks need to allow TCP through. All the steps down to the transport layer now require legacy maintenance.
Lots of things need to happen for HTTP/2 to 'stay alive'.
Your use of the future tense does not convince me.