It's not really a problem. Sure, Kson and Mikeneko are doing good financially seemingly. But they're doing it outside of Hololive's umbrella and connections. Without the support of Hololive, its incredibly hard to make your original songs, make proper covers, collab with public entities and have big sponsor events like with Lawson. That's not to mention stuff like dancing and singing lessons or the trickle down subscribers effect of Hololive.
Hololive does insane marketing within Japan that the girls benefit heavily from. Stuff like members going on TV, sponsorships with convenience stores, truck decorations, being plastered on Akihabara.
Mikeneto and Kson do not benefit from this anymore. And thats before I as a westerner try to imagine the Japanese stigma around staying around in an industry you've been fired from. I wouldn't be at all surprised if Rushia gets rigging and model from the west, unlike Kson who used the same artist that drew Kiryu Coco.
Mikeneto will probably not reach the same level of subscribers as what Rushia had when she was fired.
Merch is easy enough to sell, most content creators sell merch of some sort. Marketing is the hard part