Lately SG has only grown, and while I'm not sure how far that growth will continue I'm sure it's enough to say the game ain't gonna die off any time soon. Right now Skullgirls occupies some pretty unique niches: largest indie fighting game and the only one with a notable competitive scene at all, only game where the community is largest on PC, only fighting game on Mac and Linux, one of only two tag-based games still active (while being very very different from that other one despite the Marvel roots) and the only one with variable team sizes, and the lead dev is a competitive player who almost seems to be the only one left who actually knows what he's doing anymore. Nothing is really in direct competition with Skullgirls, so most of the current playerbase isn't likely to drop it and move on to something else.
However, there's still indirect competition from the fact that tournaments just don't have room to run 500 different games. So many new fighting games have come out in recent years, making it hard for the plucky little indie to get any attention amidst the sea of other games. But it looks to me like that's finally starting to slow down, there aren't very many big releases on the horizon while more games are dying off or being replaced. The only big things I can think of are Tekken 7 (which technically isn't new and is starting to piss people off with how long Namco is taking to actually release it), KOF14 (which so far has met lukewarm reception), and Pokken (which I'll refrain from ranting about right now, let's just say the LAN issue will make it a hard sell to TOs). Meanwhile a lot of older games are dying down, UMvC3 is gradually declining from center stage to side game due to Disney stabbing it in the back, ArcSys is supposedly retiring BlazBlue after CF to focus all their eggs in the Xrd basket instead of trying to juggle 3 games, Mortal Kombat drove off a large chunk of angry players by openly shitting on PC, and Capcom is doing their damnedest to make sure USF4 dies - a nice gesture to free up room for the rest of us. Not to mention all the controversies around SF5 - hard to say where that'll end up in the long run, but for now it seems to be driving a fair number of people to at least go try out something else for a change.
Even with less competition from new releases though, I'd say there's still two things we have to worry about. Now that development has mostly wrapped up, it may be hard to keep bringing new blood into the game, since the kids these days want their games to be constantly changing in order to hold their attention span. And like it or not, Evo dictates what a lot of players will pay attention to, so by turning us down year after year they've imposed a hard ceiling to limit our exposure. Growth may be difficult from here on out.
IMO, I think we'll continue growing for a little while, then hold steady as long as there's nothing to replace us with. We're never gonna make it to Evo main stage and some people may say Skullgirls is past its prime, but I just don't see any threat of the game dying either. In fact I expect it'll outlast many other games due to all the unique things that set SG apart as that li'l indie game with a heart of gold. Maybe that's just fanboy bias talking, but I got a gut feeling we'll be alright.