Yeah, you never watched cartoons. Some cartoons that were good enough to have long series eventually ended because that's what TV shows generally do. Your logic fails again.
I have watched cartoons ever since I was a baby. And you're statemnet doesn't make sense to me. Generally a cartoon doesn't die beacasue it just does:
Reasons
1) Either the creator left the show and then crappy new episodes came baecuse of that, meaning ratings fell.
2) Not good ratings
3) Just plain stupid and dull, meaning, again, less ratings
4) Creator died
5) Creator runs out of ideas for new episodes, cancels it.