i would prefer defiance to remain a niche MMOTPS. yes ofc having a healthy playerbase is vital.
why do i say this ?
lets look at WoW in the early stages of its life. it started of as a niche that only pc nerds played (and it was the best time ever

) then it became mainstreem got sold to activision and become the horrid panda loving game it is today.
back when it was a niche game (vanila and TBC) was alot of fun, then by the end of WOTLK it got bloated with casuals, and cata reasigned the game for casuals. and now panda expansion (do i really need to say more).
so all in all a niche game caters to its playerbase to keep them, mainstream only cares for more and more and ignores its older (and perhaps founding) player base.
i for 1 hve wanted an MMOTPS for a very long time that is PVE focused, if i want to kill players i just play BF3 /TF2/space marine

.
also look at guildwars 1. how long that lasted as buy to play + cash store