Ending solo/PG would kill the game off almost completely.
Or, for those that won't quit, they'd just set up their router to block instancing.
Overall, you'd end up with equal, or less targets... Lol
A good amount (no official numbers) of the player base still want a solo experience, like the original games, no other human players around.
Many (I assume) want a multiplayer PvE co-op sort of experience (Mobius type players).
Others want a mixed experience of PvE and PvP.
PvP only players are a small minority, that is undeniable.
So setting up the game to only please two of the 4 groups would be wrong, and alienate atleast half, or maybe even more, of the player base.
(There maybe more than 4 groups, but I can't think of any more!)