I think people are taking the metaphor of "pillars" and "foundations" too literal, and it's only confusing conversations because different people mean different things.
I'd rather talk about "importance", generally, and specify that with at least two axis, like "important, meaning high view count, public popularity, etc." and "important, meaning behind the scenes leadership, group cohesion, networking, etc.", so we can better assess and differentiate the different impacts of their potential graduations.
Say for example Fubuki, she's not the highest watched Holo by any stretch (high subcount notwithstanding, many "dead" subs), but rather average in this regard. But she's certainly very important in other more intangible regards, being a support leader, networker, group cohesion, moodmaker etc. (and selfdescribed "canary in the coal mine", so losing her would have that added special weight).
On the other hand, someone like Pekora (and to a lesser degree Korone) certainly gets most of the highest numbers in Hololive, but socially? They're rather isolated, or at least don't really have that "indispensable" quality about them that'd make the whole group collapse if they're gone.
Of course, it's a two-dimensional axis, so some Holos can have more of both qualities at once (high numbers AND high social value, like let's say Marine for example), while others are lower on both counts (for example, and no offense to either of them, they're both lovely, but the company could manage losing someone like Aki or Mel rather cleanly without much impact, compared to others)
