Well I can't speak for the rest of these anons, but usually when I watch the series finale of a show. I expect it to have the protagonist, ya know the main character, be at center stage regardless if there are no personal stakes or loose plot threads surround his/her character arc. Yes Finn found other humans, saved Ooo repeatedly time and time again, loved, lost and loved again throughout the life we've seen develop since 2010, as a character, he is at peace with himself, and his place in the Multiverse just existing with his friends and family.
However, that does not give you the right to just say "Oh well, since there's nothing left to give him to do, lets just push him to the side for the rest of his supporting cast!"
What Adam Muto and the remaining writers did to the Adventure Time finale was egregious and insulting to long time fans. Fans who yes, enjoyed the shipping for a time, but did not wish to see their boy, whom they practically grew up with and learned from his adventures, get the literal shaft along with Jake just so everyone else could save the day and have their plot threads hastily tidied up. It was lazy, pandering writing that caused the Adventure Time finale be what it is seen. Writing that basically says "Fuck it, lets do this because we can and forget how this goes here and how this makes sense from a narrative Standpoint". Writing and Marketing that has your boy be center stage on the promo poster that lies to the audience that he matters, what he accomplishes in the finale matters to the story we are about to be told. Writing that blatantly ignores previous episodes that hyped up to this moment just to subvert expectations.