>>149005177 (OP)The show still had enough of an audience to keep going, but it was 7 years old by then. Younger kids had newer kiddie cartoons to watch, older kids had newer shows like Batman and X-Men that were more 'serious' and 'mature' from day one, and were more popular by this point in the 90s. Trying to change TMNT to compete with them was always going to be too little, too late. In retrospect it probably would've been a better idea to just end the show, wait a few years, then relaunch with something more serious in tone, closer to the original comics, but that would need them to take a loss now in the hope of a possible future gain that may not even work out, so you can see why they did what they did.
Watched out of the context of the time period it was in for action cartoons, it's just a jarring shift, and the main villains being dropped and replaced with an OC isn't a great move, as it was one of those cartoons you'd watch for the villains as much as for the heroes.