It may not be just the tone of the new cartoon that puts off older cartoon fans, it also the lack of any of the familiar characters and story elements from the old show(outside of the main characters), the 1990 movie was a combo of the cartoon(skateboarding, pizza, shell jokes) and comicbook(darker tone, plotlines), the new cartoon is mostly an adaption of the Mirage comic.
Getting back to the subject of shows that was seen by more then one generation, what about;
Tom and Jerry(1975-1982)
Alvin and the Chipmunks(1983-1990)
The Muppet Babies(1984-1990)