Some use cases don't really want TCP or a TCP clone, for example multi-player games might only use a semi-reliable loss detection system which has ACK's but does not attempt retransmission on failure to deliver. This works a lot better than TCP for that use case, and is not really a "TCP clone".
The lowest common denominator is users behind an NTLM proxy. Mass market products don't generally work unless they have a fallback which is "I can make it tunnel through proxies". Which is HTTP or CONNECT :( Which means no teams will do something which isn't TCP anymore, since it's not a thing they can tunnel :( Interested in counterexamples though.
This is the same reason why a UDP replacement is a great idea but a waste of time for most people developing a product. This makes me sad.
They don't want non-web apps to work. Those networks deliberately rule out networked games, uTP, VoIP, IPsec, WebRTC, etc.
If you want to worry about people teetering on the edge of the Internet, you should worry about people behind NAT and its prevalence to the point of people confusing NAT boxes and routers...
(Re your last sentence, it's not a UDP replacement, see other comments.)