Yes... Seinfeld. However as far as the PBS network goes, the nostrums still hold - out with the old, and in with the new. For long runners like Arthur, older episodes are weaned out as new episodes are available. This has also happened with other long running programs on PBS (Thomas & Friends, Sesame Street).
It's worse for programs that have been cancelled, which are typically retired from the schedule after they end. They may be reran for a while, but usually no longer than 2-3 years after they end. It also costs the PBS networks to renew the rights to air any program - and some parent companies are very stringent these days. That said, exceptions do exist - the local PBS station ran the old Bill Nye TV show last year.
That said, I can't find any boxed DVD sets of the older Arthur seasons. I'd be more interested in prodding PBS to release them as full sets, as that way the episodes can be available at least.