Yeah the theory is that if it's not boring you shouldn't use it. It was disproven. There's probably a better heuristic model out there that takes into account

Yeah the theory is that if it's not boring you shouldn't use it. It was disproven. There's probably a better heuristic model out there that takes into account developer ergonomics, time to mvp, etc. And no, theories do not need measurable data. Qualitative results can be used to support theories (much of my hard science grad school work was basically a binary "can this happen"? That is not really a measurement in a meaningful sense unless you consider all observations measurements -- I do not).