IMHO, your point is valid: currently, it's not possible to enforce a constraint on `format!()` arguments, AFAIK, so we cannot say that all arguments to that format string, which will be used in that API, must implement SafeHTML trait or SafeSQL trait. You should create a ticket or RFC for the problem.
I'm not saying that format strings necessarily make injection vulnerabilities more common, but rather that, when added nowadays, they should be designed in a way that makes such vulnerabilities less common. If you're adding a feature that makes a potentially dangerous operation more convenient, you should also make it safer so that the convenience will help pull developers toward the safe option.