Those companies do things that you have to be big to do. Other than marketing, you can get everything you need to vtube within one person's credit card limit; with the current market new corpos spend a huge amount on advertising, but that's partly because there are still hundreds of them.
So the only way I can see a handful of big corps dominating vtubing is if viewers decide that's what they want to watch. Which is admittedly all too plausible.