Well, that conclusion is hardly surprising from you, since you had already reached it as a general statement when you asked the question.I think you are holding an impossible evidentiary standard. It’s be like ignoring all evidence in a criminal trial unless there’s video evidence of the crime...
But as a reminder your question was "What evidence would convince you that a class is extremely underpowered at combat when not taking any combat ASI's or feats?"
So, now, just wanting to point out that it was not about, "underpowered", not about "weaker than the others", not about "in need of a tweak or up-tick" but "extremely underpowered in combat" and the word "extremrly" demands a higher degree of a lot of evidence.
I mean, simply put, if a "class/sub-class combo" was extremely underpowered in any pillar, wouldn't you think it would likely show up in actual play, outside of white rooms and excel spread sheets in the ways I described? Given that different classes enable the player's choices to bring different strengths and degrees of strength to each pillar, an "extremely underpowered" should be observably so in actual play in the ways I described, right? Not just on some bookkeeper's ledger of assumptions, not just at a few tables here and there with some specific table-factors spotlighting it, exacerbating it?
So, for your real world trial evidence metaphor "it would be like..." I would describe it as for a capital crime (vis-a-vis extremely underpowered) I would liken it to insisting on actual evidence someone was dead, something tangible, observed and established by witnesses or forensics or ideally both. Especially for a class which keeps showing up as one of the more often chosen in actual play?
But just to be clear, in your "real world" metaphorical "it would be like..." you are aware, aren't you, that video evidence of a crime in not all that uncommon, maybe at best rare but certainly far from an "impossible evidentiary standard", right? You have heard of that happening, right? With more and more cell phones and cameras, it's more and more common - not an evidentiary impossibility.
Perhaps you define the word "imposdible" or the word "extremely" in a more unique way? But taken as is, your not surprising reply and pre-clusion seem a tad odd.