When the series finally ends its run, there will have been at least, nearly 250 episodes aired (though I know many earlier episodes may never be rebroadcast again due to outdated technology and pop culture). I bet, with so many episodes available to PBS's disposal, I bet the show will likely continue airing on PBS in indefinite reruns for some years to come, like is the case with "Clifford". It depends on how long PBS's contract to air "Arthur" runs for. I find that, whenever a show ceases production, PBS airs indefinite reruns for a few more years, supposedly until the contract expires. Contract expiration plus lower ratings may have led to the demise of "Reading Rainbow" on PBS in 2009.
"Clifford" ceased production by 2005 (during the "Puppy Days" format) but there were five seasons combined covering both versions of "Clifford". I personally think "Puppy Days" replaced the original version because of Clifford's voice actor John Ritter's declining health in 2003.