Give the SF2 source code to that same community today and in 2024 I'm sure we'd be astounded as to what "SF2.5" could be. However, that's not going to happen. Even with depressed sales TK is still making SOME money off it all, and there's no reason to dump that.
Perhaps if at some future date he sees no sales for any of it for several consecutive months he might consider it, but even then he's worried about people using his work for their own monetary gain. He's still leveraging many of those assets for the mobile products.
SF2 was too big for one man, but TK never wanted to believe that. I think he DOES believe that SF3 would be, so it's not going to happen. I get the impression that TK's #1 trait is independence. He doesn't want investors because it's not much different from his old relationships with publishers.
He wants to do his own thing and make a living doing it. If it doesn't, he changes what he does (mobile) so he can. He's not going to partner with anyone and at this point I can't see a SF3 unless he does. Ergo...