It's not obvious to me that replacing TCP really is harder than designing your "own" chip. Scarequotes here because those graviton chips (that's what you're referring to, I think?) are of course ARM chips, so they're not designing something fresh; they're adapting a very mature design to their own needs. In terms of interoperability, a custom chip based on a standard design is probably a simpler, more locally addressable problem than new network protocols.

Isn't it plausible that graviton was designed yet TCP retained simply because graviton as a project is easier to complete successfully?