This argument assumes that they’d only do something if they could get perfect coverage, which isn’t very compelling for me. IMO the question should be “would it give enough access to more information to be worth it”, not “it’s only worth it if it gives access to all information”.
And, as the other commenter mentioned, it is difficult to get some Chinese phones, though not impossible and if this whole line of thought plays into that, the reasoning is probably as much about cutting off their access to this kind of thing as it would be about making it harder to avoid western agencies doing this. They’ve said the first one out loud (they being politicians justifying blocking Huawei), and wouldn’t have said the second part either way.
Or give a 900 area code.