Sure it is massive. Perhaps not in game world size, but in playerbase. And if you take an mmo, in common speech, you might find what? 100ish players in a certain area? Only 12-20 in an instance. That's pretty much in the same levels of a regular shooter or a huge open area shooter with 62 people in a server.
I do have to contest persistence in most mmorpgs tho. You save the world from the same guy how many times? How many of the same monsters you kill that simply appear allover again without ever facing any extinction? They only save your character stats, maybe some guild city and guild stats, but that's pretty much it. Unless we're talking about wurm, but that is another click-grind-fest. Wurm would have been oh so much better if it wasn't for that. The very assumption they had to make people grind to stay in the game was exactly what set me away. Same with wow (yes, I have played it), I got to level 40 and the quest grind was so bad and slow I left. Same with CoH, after grinding a few heroes all because I wanted to see how they would play with all powers and then facing the stupid fact that there was no pvp, no fun, only more item grinding at the end just put me away again. If the process was filled with fun pvp and actually non-grindy stuff to do, like original player-made quests, developing your own enemies, sharing them, maybe even be super rewarded for helping new players level up, etc. You can add a lot of fun things to this list that any mmorpg would say, "WOW, not, they would be top level too fast and leave!" and sadly, those things would be the things that would keep me in them.
Skill doesn't always mean twitch. It can be slow paced, it can be chess, it can be turn based. It doesn't have to be counterstrike, even if both are nice examples of player skill requirements.
Play warhammer online when you can, it has decent pvp. Or guildwars. Or quite a few others to be honest. The problem is making pvp an end game thing.