These are the 4 pillars of Hololive. It won't always be this way if they continue to grow but as it is right now, the company cannot afford to lose these specific Holos. Hololive thrives over the competition because of the family aspect, making watching Holos more akin to watching a long running anime rather than a bunch of individual talents. These 4 are each centers of very important and intricate emotional support networks that the girls rely on to not lose their mind and stay motivated.
Gura makes the cut mostly because her mere presence legitimises Covers business model to the entire western Youtube world, which is filled with many who openly wish that " Idolfaggotry" and the " acting like cute anime girl" aspect of Vtubing would die. So long as Gura exists they do not have a leg to stand on. And so for the sake of everyone else she must not fall.