It's insane that lightgunners are even being compared to fps at all. Wii's the only console where the genre ever managed (briefly) 800k-1m+ for several entries, it's not like this this is an inherently huge genre that's always moved those sorts of units. It certainly never did on PS2.
It simply wasn't sustainable, especially for $49 releases (which the majority of previously susccessful releases weren't even). I mean, we're talking about a genre that peaked in 90s arcades and a general entry can be played through in an hour or two. On top of that, now even the "core" base on Wii (whatever that means) seems to be rejecting these titles out of principle. If you can't deliver a "real" Resident Evil or Dead Space, then don't bother...