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.