But they have internal playtesters, focus groups, other ways to acquire data.
I don't see any evidence at all that they have significant use of focus groups. They should do - they should have tons, but they ever talked about them? They're easy to spin very positively.
Looking online, the only thing I can find is that WotC may have done focus groups
on D&D players - but not about D&D - about cross-marketing synergies - i.e. what other brands D&D players think are cool.
Internal playtesters, I'm sure exist, but if the material we're getting in the UAs is the result of internal playtesting, I have to say, I think they're probably not very helpful lol. And probably a weird bunch.
And what other ways are they acquiring data? D&D Beyond? We know that, but according to WotC's own accounts of this process, they're pretty much relying on the surveys and nothing else. I'm not actually sure how much useful data they could derive from Beyond. Some I'm sure, but to inform the design of a new edition? Hmmm.
That's probably why the threshold for change is 70%
No, we know why it is, Mearls said so back in the day - it's because it's an overwhelming majority.
And they felt that once 5E, an apology edition, was in place, that was what they needed in order to justify changing anything at all.