Fighting games in general have taken a nosedive in popularity. The big two 3d fighters Tekken and VF have both dropped off significantly for the last few sequels. I think there will always be a hardcore fighting game crowd that will buy the latest prettiest fighter but barring a fighting game revolution of some sort (lagless online, new gameplay of some sort, etc) I see the sales shrinking even further.
Getting a fighting game out at launch is usually a pretty good bet, because there's a lack of games and the person who wouldn't normally buy a fighting game might pick one up but it appears that fighting games are reaching the point where no matter when you release it it's not going to sell blockbuster numbers.
VF5 has nothing to do with its platform, it has more to do with what type of game it is.
Sega still has a ton of pretty popular characters in their stable, I'd be sorta cool if they took some of the work they did on VF5 and did a SegaSmashBrothers sort of game. It'd be a good way to re-use the tech of VF5 and I think it'd sell quite a bit more than a traditional fighting game.