WWWF/WWF/WWE has been around for a loooong time. If the McMahons had thought of WrestleMania in the 60s or 70s, it'd still be around today.
What I keep thinking about is during WrestleMania X-Seven, Bobby Heenan joking that by the time the Iron Sheik entered the ring it'd be WrestleMania 34 or something. Back then, I didn't think that would ever be a possibility. But looking back, and looking at how we're already at WM 26, that's definitely a possibility. I can see there being a WrestleMania 40, 50, even a 60.
Imagine... WrestleMania L.
Think of it this way... the SuperBowl is up to XLIII, isn't it?