It's not irrelevant. Because if you are talking about pacing, that is the main gripe of Act 2 whereas Act 3 does not suffer in pacing, but in poor writing structure, conclusions and character arcs.That's totally irrelevant to whether we can discuss the pacing and building and exposition of character's desires and motivations... and if they were explored fully.
I don't care if it's the main plot or a sub plot. And actually, it IS a main plot. It's one of the three major subplots [can we call it that?] that must be fulfilled in the beginning to progress.
A small portion of the main quest, imo, eclipses the writing for the entire third act. That's kind of an issue in quality. And just to be fair, I don't think these writers are shite, quite the opposite. That's why I'm so confused.
Having a good subplot does not make Act 2's main story any better than Act 3. How the main story is handled in both acts are just as poor, albeit for different reasons. There is not that many reasons to think the general quality of writing in Act 2 versus Act 3 as it pertains to the overarching story of the game, has changed. It has a shining subplot, which does not excuse the utter lack of focus on the main narrative.