I think it's actually pretty debatable, depending on what you count "longest running" to mean. Curb Your Enthusiasm has been on since 2000 but has taken years-long breaks. Is that longest running?
FWIW It's Always Sunny is, I believe, considered the longest running live action sitcom. Longest running sitcom overall is The Simpsons.
I can't see a critic taking that dictionary definition very seriously. If you had looked at Wikipedia instead, you'd see no animated show is discussed in the article on sitcom, suggesting animated shows are considered a separate genre.
If you opened a textbook on mass media, it might have a definition of sitcom that is more culturally relevant.
From The Simpson Wikipedia page: "The Simpsons is an American adult animated sitcom created by Matt Groening for the Fox Broadcasting Company." The sentence links to the "Animated sitcom" page, which states, "An animated sitcom is a subgenre of the sitcom that is animated instead of live action that is geared toward adult audiences in most cases. South Park and The Simpsons are two of the longest running animated sitcoms."
Not the longest running show... soap operas, and possibly some news programs, hold that title. For example, the soap 'Days of Our Lives' has been running since 1965.
I'm not sure it's our job to keep adding qualifiers to the OP's bold statement that "Simpsons is not considered a sitcom, but it is the longest running show, period." until it's finally accurate.
In any case... PBS' Nova is 47 years old. Frontline is 37. I believe those are aired in prime time. And they are "not considered a sitcom" either.
