yeah I mean the show didn't get terrible or anything. I can put on a new episode and laugh a few times. Just isn't the same at all
As for the Principal and the Pauper, I agree with what Harry Shearer (I believe) said. You spent 8 seasons developing these characters into people that the audience truly cares for. There's a Simpsons canon that millions of people followed. And then one episode you just throw it all away for a cheap plotline. Then ignore it by the end of the episode. It really cheated the audience. And I think that is when the Simpsons started to go down hill, becoming less heart and writing and more throw jokes at a wall and some will stick.
You pretty much nailed it on the head. After the episode, it really became about what outlandish situations the Simpsons or the other characters would go through next. Not to say later episodes weren't funny at times because they were, it just wasn't the same magic in the air like the early nine seasons. South Park kinda became the same way, it was a smart and funny show that became about satirizing whatever popular event was going on at the time whether it was funny or not. If anything, this episode is more or less writing on the wall to the future of the show. If it ends, I'll be bummed but at least I can look back on it as a great show that had it's low points but never failed to make me laugh.