You overlook an immensely important takeaway from the subtext: when it's asked of BMO what happened to Finn and Jake, he says "they just lived out their lives". Despite accomplishing amazing things they were just people, who never had pitch perfect nor brutally tragic endings destined for them. Did they get what they wanted ultimately? They could have. Again, we don't know and that's the point.
If you don't find that satisfactory then that's completely valid, but for me specifically as a viewer it resonated with in the way I assume the creators wanted it to. We can do good things before our lives are complete, we can make ourselves extraordinary in what we accomplish, and even so, after changings of era and uprooting of status quo, we just keep floating down stream at the same pace. It's a fact of life and our destinies are always going to be up in the air. We don't really know what happens to *any* of those characters, even PB and Marceline.
IMO a show is a living being, its achievements can't be credited to one person, even indirectly. The Simpsons had a once fresh and energetic cast with a anomalisticly talented writing crew, who knew how to play to the concept's strengths. Once those strengths are realized for a good decade there's very little anyone could do to save face and keep it going for more than double that time.
Meanwhile something like SpongeBob, albeit Hillenburg's brainchild, was what it was in its prime because of that specific combination of Storyboard artists, writers, and voice directon. Nearly all those positions are filled by new people, with different interpretations of the source material.
The creator can only put an idea in motion, once it takes on a life of its own it will move based upon the input of hundreds of other people. It really can't be helped.