Eh, I think it's too soon to hit the panic button on Tekken. There are too many different issues that seem like they are related to the core game when they may not be.
First, sales. The game sold well. Namco has not complained about the sales. Consider it was the EIGHTH fighting game released in a 12 month span of time, in an environment where only AAA tier games are moving any units, on consoles that most people are tired of playing anyway.
Tournament turnouts...this one is a bit tough. It appears to me that the usual tourney goers for Tekken are the same guys that have been going for several years, and those guys are getting up there in age. Losing their abilities, getting a family, job stuff, boredom, online being good enough (!), all of those things are a factor. But the younger crowd to replace them just isn't happening (or not quickly enough). Why? Well, aren't people staying at home more in general? Sports attendance is dropping, department stores are closing, all kinds of changes are happening. Tournaments are a labor of love for gaming, and as gaming becomes more ubiquitous in society, it becomes less a hobby of love for people and more of a commodity to pass the time. People don't buy CD's and expensive sound systems to just sit and listen to music anymore (except me ;_
, an iphone loaded with mp3s and a Bose fake surround speaker are good enough. I expect turnouts across every game to drop over the coming years unless something truly radical happens.
Is the core game too hard? Maybe, but not really. Movement and memorizing which strings jail are the only things I consider to be a real chore in the game. The other complexities make sense, but really need to be better explained and detailed throughout the game. I still don't think that would greatly increase the competitive playerbase though.
What really needs to occur is to incentivize things outside of winning. Only good players win, and most people aren't good, so if the only way to have fun is to win then most people won't have fun. Treyarch figured this out with COD, and it annoys the hell out of oldschool FPS fans, but that was successful. I think that kind of approach would be a great fit for fighting games since unless you're good you shouldn't be thinking about winning anyway, only getting better. Obviously "perform five EGWFs for a medal" isn't exactly the kind of thing I have in mind, but I don't have a real concrete idea beyond that. That's why game designers get paid the big bucks to figure it out!
As for the Capcom thing, it's best not to compare anything to them. Those games have built in advantages. 25 years of history and nostalgia for Street Fighter, and Marvel. I seriously think people underestimate the appeal of Marvel. The game is easy to pick up and play and has a strong history in the FGC, but that's still a game where half the roster has half a century of top level popularity in American culture.