> If you aren't instantiating a template, then it isn't used, so who cares if it has theoretical errors to be figured out later?

Just because you aren't instantiating a template a particular way doesn't necessarily mean no one is instantiating a template a particular way.

A big concern here would be accidentally depending on something that isn't declared in the concept, which can result in a downstream consumer who otherwise satisfies the concept being unable to use the template. You also don't get nicer error messages in these cases since as far as concepts are concerned nothing is wrong.

It's a tradeoff, as usual. You get more flexibility but get fewer guarantees in return.

> If you aren't instantiating a template, then it isn't used, so who cares if it has theoretical errors to be figured out later?

This seems like a very strange argument to me. For a pleasant experience you generally want to report errors as early as possible.

You're right but concepts do more than SFINAE, and with much less code. Concept matching is also interesting. There is a notion of the most specific concept that matches a given instantiation. The most specific concept wins, of course.