Not many pop culture Phenomenon last for more than 10 years. When I say Phenomenon I am referring to wildly corny concepts that the general population considered something they will invest money and time on. Media like Alf, Dinosaurs, Thunder Cats, etc. The Teenage Mutant Ninja Turtles have not fallen to the same fate as these shows. TMNT was created around the same time as these and, other than a few years in the early 2000's, they are still running strong 30 years later. Why would a show with a concept as absurd as turtles becoming crime fighters continue to be relevant?
The generation that grew up being into TMNT is growing up and having kids, these kids look up to their parents and want to be interested in the same things that their parents were interested in. The young teenage crowd may continue to love the turtles because they are going through the same angst as each other. Also, The hipster movement of being ironic, different and of enjoying things that seem purposefully bad would be attracted to the goofiness of the show. In the end, the TMNT are still popular because they are lighthearted pizza loving characters who bring us away from this boring world of turtles that move slow instead of fighting crime.