Yeah, there’s a ton of games that are very popular where PvP is all there is. The hook with MMOs is character development. Players like to see their characters become more powerful. The problem I see with character development requiring PvP is that there will be winners and losers, and losers end up unsubbing.
It wouldn’t be hard to put in boring, static PvE. Just go old school and populate an area with monsters who aggro and give XP when killed and respawn.
Shadowbane had some very basic PvE. It served to help players level and learn their character abilities.