Turning completeness doesn’t take efficiency into account, nor the reality of things like “call into the operating system so that you can display output” that are necessary when building real systems.

With respect, I think that's moving the goalposts. If we have defined out of existence all forms of I/O, then what are we actually discussing?

We haven't, that's why Turing completeness is not relevant for the question at hand.

I can implement the non-IO parts of Brainfuck with safe Rust, so it is Turing Complete. That doesn't change the fact that there are useful programs not expressible in it.