I don't know, AT certainly took a tumble and lost a decent chunk of its charm, but I still liked it.
I think the problem is that it was certainly better at being a wacky, silly cartoon with a tiny injection of depth, than a cartoon that's much more often trying to be deep and serious and interconnected all while maintaining that same feeling of wacky silliness. Sugar helped with it, but I think her leaving was a minor thing that didn't tip the balance all that much.
It's kind of a meme saying AT got good recently but I think it is the truth, I feel like seasons 7 and 8 are a much better and more refined and thought out version of the show that season 5 and 6 were trying to be. I certainly don't think AT was ever soulless or insincere, but there were lots of parts were it was awkward, boring, and dumb, and I think recently the show has gotten a better handle on those things.
The ending scares the shit out of me because they could go ten different directions with it. My one hope with it is that they do something outside the box, because ending the show with just Finn and Jake on a hill and Finn turning to Jake saying "You know Jake, we really were a Adventure Time" or something of that sort would be dull as dishwater.