Asserting that it's impossible or meaningless to measure the quality of a UI is to deny the entire field of UX research. Yes, it's subjective, but you can meaningfully measure the average across many humans. And that's exactly what you should do if you're trying to decide on a UX that will be used by many humans.
Rather than it being meaningless to measure the quality of a UI, I meant that even a UI that tests well generally might not be the best UI for a particular person.
Sure, but if you measure that for a ten thousand people (controversially, ten thousand people that aren't already programmers may be better) you can get a good idea of what is efficient and what isn't.
Maybe, but also maybe not. Ten thousand people giving ten thousand uncorrelated responses is also a possible, not unlikely result of such a test. There's also the question of whether or not your methodology for "measuring" what is effective and what isn't is even possible, let alone sufficient for a definitive conclusion.
