@paco 💯
It is perfectly reasonable to say "not that" even if one does not have a good solution. But that has to be related to some specific bad outcome.
Saying "not that, just because I don't think it would work, but no I have no specific bad outcome I am concerned with, and no I have no alternative" is what gets me.
Esp. when it's delivered in an authoritative tone of "this will never work."
Again, if "this will never work", why even bother opposing it? Just sit back and enjoy being right!