I'm actually not entirely sure I agree with this sentiment on the vtubing industry.
Interest in V-Tubers lives and dies on the attachment people have towards the members they love, and it's very difficult to replace them with new members when they're gone. With every graduation the industry shrinks, because a portion of that person's diehard fans may choose to leave the hobby behind or, in the case of corpo graduations, follow their indie career and leave the corpo behind. They won't watch other V-Tubers more, they'll watch V-Tubers in general less because their attachment isn't to the industry, but to the individuals that just-so-happen to be in it.
There's also a lot of onus placed on certain members regarding how they affect the culture of the company. Monetarily speaking, Melissa Kinrenka graduating isn't going to hurt Nijisanji; she averaged about 600-900 viewers and wasn't really raking it in with SCs. She is, however, a huge cultural loss on Nijisanji as one of the company's figureheads in the singing department.
A Nijisanji without Kuzuha, Kanae, or Mito, or a Hololive without Pekora, Marine, Fubuki, or Miko would be absolutely devastating to how fans respond to the concept of V-Tubers and V-Tuber companies. Lain Paterson gets more viewers than Shizuka Rin, but it'd be Rin's graduation that is felt harder by the fans because her status as JK Gumi means so much to so many.
Kizuna AI was always innovating her tech and yet her numbers dwindled. The core of the V-Tuber community will always be attachment. The industry might continue to explode in growth, but there's still those few members who hold a lot of power over how people perceive the industry or the company they prefer.