I know around the same time Cinar got busted for that tax fraud scandal is when former head writer Joe Fallon also walked away from the show (some speculate they had to let him go, as he was among those "foreigners" being passed as Canadian citizens, as he is originally from Los Angeles); I also know that because of that, they dropped "Crazy Bus" as D.W.'s favorite song, because it was Fallon who not only wrote the song, but performed the vocals as well.
Either way, I kind of felt like this was the first major blow to the show, because it seems like most of the better episodes from the first four seasons were coincidentally written by him... in a sense, it was almost kind of like he did for Arthur what Jon Stone had done for Sesame Street, there was such a spark of creative and imagination that really isn't there anymore... that's not to say that the show hasn't had good moments since then, because it has, but just not quite the same level that can measure up during the years Fallon was writing.