It's a common misconception that routers handle TCP. They strictly handle only the IP headers (and lower-level headers).
The TCP protocol is implemented only by endpoints, at least in principle.
It's the "security appliances", also known as "middleboxes" that are the problem. Think web proxies, antimalware scanners, firewalls, and inline IDS systems.
These things are the bane of the Internet, because they ossify protocols, blocking any further development.
Although what a consumer considers a "router" is actually a middlebox doing a bunch of things and does care. (CG-NAT in provider networks is probably another example of a common problematic middlebox)
“A new version of TCP” is pretty much what QUIC (basis of HTTP/3) is. It’s just tunneled over UDP because existing Internet infrastructure likes to drop anything that’s not TCP or UDP.
Depends what you mean by "real". You may know this already, but the only difference between UDP and raw IP is the UDP header, consisting of 4 fields in 8 bytes: source port, destination port, length, and checksum. That's it; there's no other protocol overhead. Thus, from a pure technical perspective there would be basically no advantage to running QUIC directly over IP instead of over UDP. The only advantage is from a human perspective, that it's a little more elegant to put QUIC on the same layer as TCP.
In exchange... among other things, it would break all existing NAT implementations, since NAT is based on port numbers and existing devices wouldn't know where to find the port number in the new protocol. So everyone behind a home router would be unable to use the new protocol until they upgraded their router firmware – which of course most 'normal people' never do, so realistically you're waiting years until they get a new router.
Not only is that a gigantic practical disadvantage, it also feels rather inelegant itself. After all, routers shouldn't need to know the details of the transport protocol just to route packets. If it weren't for NAT they wouldn't have to, which is probably why port numbers aren't part of IP itself. NAT sucks. But NAT isn't going away; even on IPv6 some people insist on using it. By tunneling QUIC inside UDP, we at least regain the elegance of separating what routers need to know (IP + UDP) from the real "transport protocol" (QUIC).
Except we already have those layer 4 replacements (i.e. SCTP covers a lot of the same ground), and they've never managed to get out of the niches they are in. How would you suggest "a major internet company" motivate their support better?
It is about the same. It is called TCP/IP for a reason. Although there are more devices that deal with TCP/IP together than just IP alone. Either way it ain't going to happen.
IP doesn't require new hardware because it's a lower layer.
Transmission Control Protocol - TCP - is baked into the firmware of every client network interface card, and I would suppose in almost all of the switches and routers of business infrastructure.
I have no idea what data centers use. Infiniband and similar things aren't TCP, I think.
