Is there actual evidence he was responsible? I'm not saying you're wrong, but if he took over in February of 1996, it had a delayed effect. Rugrats went to hell in the episodes made after David Doyle's death (97). The first of the newer episodes in 97, with Doyle present, still had the same magic as the older ones. Even the last episode ever made with him ("Lady Luck") was well-crafted.
With Ren & Stimpy, I think there were problems with the creator/head writer, who didn't get them completed on time and was in conflict with network execs. It's one of the reasons R&S is the one nicktoon without the standard 4 seasons/52 episodes catalog that they had signed on for (well, Rugrats and Hey Arnold exempt, since they were so popular they were given additional seasons).
I don't know the ratings figures for the shows, so I can't really comment on the rest. I do know Alex Mack was extremely popular, though. Personally, I thought it was about time to start wrapping up simply because of where they were in the story (let's face it, they couldn't keep it a secret as the actress started getting closer to being of college age), but it may have been more a matter of fixing what ain't broken..."NEW SHOWS, to infinity and beyond!"