That's not quite true. You can't make any assertions about the behaviour of the system if only one part of it is verified. The way to verify the entire system

That's not quite true. You can't make any assertions about the behaviour of the system if only one part of it is verified. The way to verify the entire system is not to verify that quantum physics works; that doesn't help. Instead you have to assume that the logic gates you are using are deterministic which is impossible to prove. Then you work up from there. Despite this, I do agree that having part of the system verified is better than having none of it verified.