as far as i am concerned they could go on forever.
think about it, "i" dont think they need to move onto new hardware. i like the graphics in last blade or rbff2 as much as those in sf3 or ggxx (yes i am aware they are not as "technically" advanced), the reason why being that 2-d fighting games dont really need better graphics to get the job done. yeah you could get onto a different medium and have 60 chars and a 120 frame rate but the fact is a 60 frame rate and 30 chars is fine, actually pretty much ideal.
move onto new hardware and the desire to make 3-d games just to sell to the masses and be cool becomes too strong.
2-d is not old skool and 3-d new skool, they are two totally different genres which are both worthy of support. i personally get no satisfaction out of playing 3-d games competitively so....
besides games like tetris and castlevania are still around and played by many, they dont need graphic overhauls... the overhaul should be the games quality, system and creativity not its graphics or a move to 3-d... thats how i feel anyway.
?