Why casual players don't 'just leave'

[hydralus]

Aksually.. Genshin is not an MMO

other that that, the reason why MMOs are "dying is because"

1. Trends change
2. MMOs are not profitable enough to make them compared with other genres (takes over 10 years of development). They prefer to milk the cows/whales while doing gacha games that they can push at a much fast pace or re-skin/ re-use a lot of the existing assets.
3. It's much easier to "train" a young person to become a regular spender than somebody who grew up with old-style MMOs that only had skins as a form of monetization.

Every single MMO that attempted to implement pay-to-win, gacha, or heavy monetization into its model simply died out really fast.

Which is funny because SE has failed in most of their attempts to monetize mobile games.

Also D&DOnline introduced pay to win when it moved to a free to play model and it's kept it going for over a decade.