@Yuriglutton : It's not prostitution and it's a lot older than streamers. It didn't even start with Harvey Weinstein, though he's the most popular example nowadays.
A lot of the time in entertainment, or any employment setting, a woman might sleep with someone or some people to either climb the ladder or keep her job.
It may be that her boss is abusing his power to coerce her into sex for her to get promoted or to keep her job and not be fired. It may also be that the woman herself is using sex without coercion to secure a raise or promotion. If you want to have this rather depressing conversation, the MRAs and feminists would be more than glad to destroy your head with it.
This could be the situation Umi presented Mizuhara with on the talk shortly preceding her grabbing Kaz and leaving the party. Mizuhara as an individual doesn't seem like the type who would go within twenty miles of this. She's talented, beautiful, and resourceful, and in her case, if presented with this situation, she'd simply walk. As for Sumi, the sort of girl she is, if she even smelled the prospect of sexual coercion in a job, she'd never go near it.
With rental girlfriends, the structure of Diamond as a company seems to draw a lot more on the Japanese geisha. I am by no means an expert on this, but they seem to be experts on companionship and conversation. They may sometimes choose to sleep with the client, but rarely, and that is not the center of their job. Japan's attitude towards this is weird. The documentary I watched had an elderly couple being interviewed together, and the wife literally said to the camera: "If he slept with another woman, I would be furious. But if it was a geisha, I would be okay with it." Her husband was right there. A geisha's client may provide her with something as lavish as a condo -- and the geisha, due to her profession, will never disclose who it was. It seems to be a very high skill job needing class and discretion. So, I think that's where the whole rental girlfriend comes from.