It's not that TCP can't be changed, it's that it's the only option that works.

TCP is baked into firmware on edge routers and firewalls and wifi devices that are beyond the period of support and the companies that created them aren't interested in supporting them or have gone out of business.

There are other protocols in layer 4 that could have been used that are better on some workloads than TCP, but TCP and UDP were the only things configured/supported; many firewall default configurations filter out things they don't recognize so anything new is a non-starter. And because of the perception of responsibility when it comes to networks, if I support a new protocol and you can't connect to it because there's older devices between you and me, it becomes my problem.