Here's a few more things I've learned:
It's now been confirmed that Season One has been permanently retired and will no longer be aired on PBS, because the episodes are, indeed, considered "too dated" by today's standards - particularly with Technology Marches On.
I also recently learned that there was going to be a Halloween special about ten years ago or so, but it was ultimately shelved since ARTHUR'S PERFECT CHRISTMAS and ARTHUR, IT'S ONLY ROCK N ROLL weren't really successful.
The reason why the episode count per season was dropped to ten by Season Four was because of a new standard that PBS had of wanting a certain number of episodes in rotation; it was also decided by this point, the writers had already tackled all of the specific issues and themes were needed to, and this is also why Arthur has been put more and more out of focus and other characters have been given the spotlight more and more.