Adventure Time - I can't stop it

♫ Everything stays
Right where you left it
Everything stays
But it still changes ♫

I completely disagree. The point of that song was to show that however things look the same, they still change, even if you can't readily see it. Finn choosing not to take a relationship with Huntress Wizard further was actually a sign of him growing as a character, not regressing. In earlier seasons, even though PB rejected him, he held onto the crush for quite a while. He made Flame Princess fight Ice King for pete's sake. 3 times, in fact. With Huntress Wizard, he accepted her wish, even echoing the spirit of the forest at the end of the episode, "Attractive forces come and go".

Jake grew into his role as a dad in the episodes that focused on his kids, and there are also subtle hints he is accepting his eventual death. (He literally sees death in a field during his song at the beginning of Flute Spell.)

As for Stakes, Marceline confronted her past, but this time she wasn't alone, and while she did end up as a vampire again, she also came to terms with it. It was also a great excuse to show more of PB and Marceline's relationship. So yes, while Marceline didn't change much as a character, she at least is now content with who she is and has friends to back her up.