While I like both ninja turtles cartoon series, I don't think they could have lasted 500 episodes each. The first ninja turtles series had 10 seasons and 194 episodes, while the 2003 series had 7 seasons, 156 episodes, and a TV movie. In my opinion, I think that both series ended at the right time, and had good conclusions.
Also, I don't think that most series can last up to 500 episodes, unless you want it to turn extremely stale.
As for series that I think were cut short...
Ruby-Spears Superman - This show only had one season of 13 episodes, when it aired on CBS, in 1988. I liked the episodes this series had, and I think that it should have lasted for another 2-3 seasons.
Megas XLR - This show only had 26 episodes. I think this show definitely had potential to last longer, and should have gotten another season or two.
Super Robot Monkey Team Hyperforce Go! - This show ended on a cliffhanger. They should have given this show one more season, like the creator had planned, to wrap things up.
The Spectacular Spider-Man and Wolverine & the X-Men - Both shows had 26 episodes, and ended on cliffhangers. Both series should have gotten another season or two.