Where to begin. A purely vertical progression system is based on scaling pretty much only, meaning first you kill a 100HP creature with a 10DMG bullet, later levels you kill a 1000HP creature with 100DMG bullets, rinse repeat. Yeah, i could try to do this indefinitely, i could fool myself into believing this is actual progress. I also could not.
There are alternatives. The perfect "MMO" would be a virtual home, another reality you spend the other time of your life in you don't know what else to do with your life. The power fantasy works in single player games because it's a single player game. You're playing it alone, it is a fantasy in its purest form.
When you introduce other players, you start moving away from this fantasy and closer to a reality.
Similar to how in your mind you have the perfect answer for all the problems of the world until you take into account that there's other people living there. The key aspect here is that the prefect MMO is an ecosystem. Not everyone is a top predator. Nor should they try to be.
Different strokes for different folks. It is not a single player experience. I've seen this been ignored so much on the FF forums, i had to bite my tongue until it bleeds. "I want to craft until my hands fall off, but i don't like item decay".
It doesn't work that way. It's the same difference.
"I want vertical progression but i don't want the player base to only care about progression".
It doesn't work that way.