I saw this thread under recent posts and was curious to have a look. I never played MK9, and I am here now by virtue if Injustice. My background really lies down the path of old school fighters, having been on the ST train originally and following along that line ever since, never having had a real brush with any MK.
Unfortunate as it is, games do have limited life cycles. As Rude tilted at, burnout will always play in it and people do grow tired of any game over time. I haven't played ST seriously in ages, even though I loved that game to death. It was going to pass sooner or later, and there's not a huge amount that can be done about that.
That said, I want to point out that the community is not dead. A lot if people may have moved to Injustice as their new game, but that wouldn't have been possible at all if the MK community hadn't been ready to stand up, adopt it and treat it as their own. People in this community were ready and willing to fold in around it and support it wholeheartedly, and for the first couple weeks what I invariably heard was that this community was the place to go to really be a part of Injustice, and so it has been.
I do not underestimate the value of that presence, either. This community and this site have proved and invaluable resource in learning and building up my Injustice skills, and I've found that much of the discussion going on is more intelligent, active and good natured than you would find in other communities. Try finding good resources for playing fighters that are clear, well organized and not buried under mountains of dick waving anywhere else, I dare you - it's not going to be a simple task. The people who complain about this community, I think, have not seen what a bad community actually looks like. There's always going to be a few bad apples, but I've found this community to be disproportionately intelligent and genteel compared to any other I've ever been part of.
You know what that means, in the long run? I never picked up MK9, and it's too late now for me to invest much interest, but you'd better believe after my time here and with Injustice, I'll be back for MK10 - and I will not be alone.