Suisei is more of an exception than the rule, since she was grinding to be an idol from the start and for her - streaming is a side activity. However, if you look at their other most popular talents - they are balancing between streamers with "le funny personalities" and being idols.
Besides, as you have noticed - industry is bloated and, by this logic, no-one will be surprised by just another bunch of singing and dancing idols. Everyone have them. There are already corpos who, from what I understand, mostly focus solely on idol activities.
Holo strength, so far, was always in picking talented charismatic people who are good at both being entertaining streamers and being good idols / artists / dancers. Would anyone gave a damn about Kanade without her brat antics in GTA, for example? I don't think so. She'd be just another singer, dime of a dozen.
Having an interesting personality is a necessary groundwork for popularity for majority of people in Vtubing. And with how Holo, so far, managed to get people with fun personalities, it is their strength. Throwing it away, just to become another generic idol factory, would completely negate it to the point where Holo might become truly irrelevant.
Ayame coasts of her popularity as the streamer she once was, so its not exactly an exception.
We can see it on Gura example - she might disappear who few month with no explanation whatsoever and, yet, despite how illogical it is, there are always thousands of people waiting for return (myself included).