Of comic strips still running new (or at least "new") installments, a huge number are now legacy affairs. That is to say, carried on by someone other than their creator. Dennis the Menace, Family Circus, Beetle Bailey, Hagar the Horrible, Blondie, Shoe, and Gasoline Alley all continue while their original cartoonists are no longer with us. And that's a very partial list. It's not inherently wrong, and some successors do better than others, but ultimately it's a business thing.
However long we make this list, Peanuts would not be on it. Charles M. Schulz continued to work on it until he died in 2000. While you could argue that in later years he spent too much time on Snoopy and off-Snoopy characters like Spike, he never phoned it in. All along, he wrote and drew with care, introducing new characters here and there even in the late nineties. By the point where he would have had to be replaced, he had become irreplaceable.
