Honestly, I was kinda sad at the idea of Arthur ending but then I saw the quality of these new episodes and realized that it really is for the best.
I think ending it at season 7 would have been the perfect run for Arthur. They hit the 100th episode mark that season and had a great episode about it. April 9th was the last episode and would have doubled as a perfect series finale. That episode perfectly captured everything that made the series as great as it was.
That's not to say that they didn't have good episodes past season 7 but it really would have ended on top if it stopped there.
Fun fact, the S1-5 character designer (Richard Morris) was still in contact with the writers when S7 was in production. The initial draft in response to 9/11 sensitivity for that ep was for a major accident to...unfortunately cripple Sue Ellen
It's probably why Sue Ellen's section of that ep was glossed over compared to Arthur/Buster/Binky's: it would've been too morbid, so it was changed haphazardly, especially with more seasons to be made. Mr. Morris leaving due to injury/age was better to grasp, but I can't imagine what would happen if the initial draft was used...
I agree with it being a really good potential finale though, though what if an actual movie was made right after to seal it? Would've been something
Shame all we got was a cruddy CG movie. At least the Christmas special S5 was nice