While I was upset King of the Hill was cancelled, it probably was better to go out while the show was still decent than wait for it to implode on itself.
King of the Hill was a very good show, but I really don't think it could have sustained itself past when it ended. Simpsons and Family Guy make money... KOTH didn't even sell enough DVD's to keep the entire series on there... the merchandising didn't materialize (the action figures sucked. You had to wait until series 2 for Boomhauer, Hank didn't come with Propane OR propane accessories). Plus, the show was repeating itself pretty bad the last couple seasons... something happens Hank doesn't like, either Bobby, Peggy or Bill get sucked in... Hank's right about the whole thing. Over and over like that. Plus, Fox shoved them in the 7:30 deathslot, and they could only show half a season per season because it got preempted for Football (same thing that killed Futurama the first time).
Simpsons and Family Guy, as meh as the writing for both has gotten still manage to pull in good ratings and good toy/DVD/t-Shirt sales. I'm going nuts collecting the BK Simpsons Treehouse of Horror toys (Only need Kang and Bart). I'd buy the new Playmates FG toys if they weren't 10 bucks each and would just find a loophole to get Cleveland Jr. and Rallo in there (Roger and Stan Smith would be a stretch). I still manage to tune in every week and get a couple chuckles out of each, even though they're both past their prime. Yet, as much as I'd like them to end, I wouldn't want to see them go.
As for Arthur, I'm actually surprised they're seeking 2 new seasons... my channel, the PBS station that's the production company mind you, pretty much dumped it off in the early hours while it's focus group is still in school. And I never see any merchandise anymore. It's still on the air and all...