Creating a new transport protocol for use on the whole Internet is a massive undertaking, not only in purely technical terms, but much more difficult, in political terms. Getting all of the world's sysadmins to allow your new protocol is a massive massive undertaking.

And if you have the new protocol available today, with excellent implementations for Linux, Windows, BSD, MacOS, Apple iOS, and for F5, Cisco, etc routers done, it will still take an absolute minimum of 5-10 years until it starts becoming available on the wider Internet, and that is if people are desperate to adopt it. And the vast majority of the world will not use it for the next 20 years.

The time for updating hardware to allow and use new protocols is going to be a massive hurdle to anything like this. And the advantage to doing so over just using UDP would have to be monumental to justify such an effort.

The reality is that there will simply not be a new transport protocol used on the wide internet in our lifetimes. Trying to get one to happen is a pipe dream. Any attempts at replacing TCP will just use UDP.

While you're absolutely correct, I think it is interesting to note that your argument could also have applied to the HTTP protocol itself, given how widely HTTP is used.

However, in reality, the people/forces pushing for HTTP2 and QUIC are the same one(s?) who have a defacto monopoly on browsers.

So, yes, it's a political issue, and they just implemented their changes on a layer (or even... "app") that they had the most control over.

On a purely "moral" perspective, political expediency probably shouldn't be the reason why something is done, but of course that's what actually happens in the real world...

There are numerous non-HTTP protocols used successfully on the Internet, as long as they run over TCP or UDP. Policing content running on TCP port 443 to enforce that it is HTTP/1.1 over TLS is actually extremely rare, outside some very demanding corporate networks. If you wanted to send your own new "HTTP/7" traffic today, with some new encapsulation over TLS on port 443, and you controlled the servers and the clients for this, I think you would actually meet minimal issues.

The problems with SCTP, or any new transport-layer protocol (or any even lower layer protocol), run much deeper than deploying a new protocol on any higher layer.

QUICv2 is not really a new standard. It explicitly exists merely to intentionally rearrange some fields to prevent standard hardcoding/ossification and exercise the version negotiation logic of implementations. It says so right in the abstract:

“Its purpose is to combat various ossification vectors and exercise the version negotiation framework.”