Actually, Ghostbusters is becoming retro now.
Just check out Hot Topic.
I would bet right now that if they had waited until now for Extreme Ghostbusters, that it would have done better than it did.
Some old fans may have lost interest, but Ghostbusters will always gain some new fans because Ghostbusters is such an amazing, fun concept.
4 guys hop into their built up Cadillac Ambulance, looking for Ghosts to Shoot with awesome Proton Packs and Catch with awesome Ghost Traps.
That's mainly why Ghostbusters lives on. The concept and the characters developed for it.
Anyway, that's my two cents.