Do you often find yourself doing mutual recursion between functions crossing compilation units/modules? I'm not going to say it can't happen, I just don't see it as much of a problem.
If on the other hand the regular calling convention is compatible with TCO, then everything becomes much simpler, because it fits in with the existing model.
Like I said in a different comment below, these are not obstacles for TCO. The compiler can simply emit a second copy of the function that doesn't need to honor a calling convention.
>So it’s a problem when implementing the compiler and adjacent tooling
Yeah, implementing a compiler is difficult work. Who ever said otherwise? I originally responded to a comment talking about TCO being incompatible with certain calling conventions. I.e. if your platform uses a certain calling convention then TCO is impossible. That's what it means for two things to be incompatible: you can have either one or the other, but not both at the same time.