OK let’s put it this way: what is the point of IP being able to carry protocols other than ICMP, TCP, and UDP? For that matter why doN’t TCP and ICMP run on top of UDP?
There isn't one! TCP running on top of UDP would have been a reasonable design. TCP/IP is full of warts like this. The URG pointer. The conflicting length fields.
The fact of ICMP not being itself a UDP protocol caused major problems for systems programmers, because it meant that OS kernels all "owned" ICMP, provided only a baroque sockopt programming interface to like 5% if it, and required userland programs to hold suser privileges to do any real ICMP work. Awful design. And ICMP is slow-pathed by routers, because it isn't UDP.
UDP literally doesn't do anything but multiplex raw IP. Unless you're actually worried about the 8 bytes the header takes up, there's no reason, none at all, to slide a new IP protocol anywhere but on top of UDP. Again: that's why UDP was designed in the first place. You can go look this up! David Reed still talks about it!
Isn’t WebTransport more or less TCP (reimagined I guess but it’s key functionality and set of capabilities) running on UDP while also able to play well with the web’s security model.