The TMNT merchandise machine has been really strong since Viacom bought the property. My kids, neither of which watch the cartoons or care about existing or future movies, still wear the shirts, play with the toys, own the backpacks, etc. The TMNT IP is still very much alive and maintaining positive mind share across several generations.
I'm not saying that any of this explains the sudden price escalations in the comics. My general belief is that these increases are merely in line with industry trends these last few months. If people are willing to pay $9000 for a variant from ten years ago then paying $9000 for a blue chip key from 1984 really isn't a far stretch.
With that said, the premise that "nobody cares" about the TMNT property, in and of itself, is not "fact". NECA, for example, has been making a lot of money from their premium action figure line for many years. Saying that current TMNT comic book sales is representative of their popularity is no different than saying the same of Spider-Man. If Disney canceled all Spider-Man comics, 99% of the character's fan base wouldn't even notice.