Rather than changing TCP, we could just make web browsers query SRV records to see what port to connect to. Then you could host 65535 websites on the same IP address, without requiring any software in the middle to check Host/ALPN/SNI. (The web is moving to UDP anyway with HTTP/3. Not saying the web is the only important part of the Internet, but it's a big one.)

IPv6 is kind of over the major adoption hurdle of rewriting all software to understand it. Nearly all software understands it. I'm not sure anyone really has the appetite for that again.