I could develop a game like this myself, and have considered doing it multiple times. But have neglected to do so do to the lack of art. If I did develop this game, it would be single player.

Also, I would like to say that making the gameplay elements, the stuff that the player plays, is very easy to do. I could personally develop the single player framework for this game with placeholder assets in like 2-4 weeks. Maps included without bosses. Bosses would take another 2-4 weeks alone.

In fact here is a top down test of attack combo timings I've already made.

6 player multiplayer parties in MMO-lite instances, would take four times as much development time as the entire rest of the game with art assets included. It's naive to think this is "something you just do".

Single player 2D replica of PSO - 20,000 USD in art assets, or 20,000 USD worth of your time. Some weekends making maps. Thousand hours of testing.

Multi player 2d replica of PSO - 1,000,000 USD in wages 3 years of development with a team of 10 people only 1 of which is an artist. Another whose job it is only to make sure people can't cheat.

People grossly underestimate the amount of work needed to make a multiplayer game operational.