I personally think "Arthur" has gone downhill for many years now. I used to actively watch the show for all of the first ten seasons. I feel as if my interest in the show was falling out of favor during the milestone tenth season. When we entered Season 11, I watched the first half of that season twice, and after that, because of excessive reruns and the content becoming quite dry, I gave up on the show for good sometime in July 2007.
It is evident that the show has been going downhill after Season 19 or so. For the longest time, one season would consist of ten fresh episodes, but today, the number of episodes in a season is reduced to just seven. This implies that ratings have been in decline for a number of years, and it would make sense, especially since kids these days may find computer games more interesting than kids' shows.
Whatever the future holds for "Arthur", it will be the longest-running animated series designed for children to ever air on television (unless, in the distant future, something better takes over). "The Simpsons", which is the longest-running animated series in television history, doesn't count, as it is targeted to adults.