There was never any fucking way in Hell either Bubblegum or Marceline would EVER have ended up with Finn no matter what you dumb faggots because it would be seen as PEDOPHILIA. Some people point to Bubblegum briefly turning young to counter this, but that only spotlights that they were both immortal adults and Finn was a literal child. I'm not saying that's fair or shipping is ever a crime like fucking tumblr thinks, I'm saying /ss/ has rotted your brain and it doesn't occur to you that anything that smells of kiddy fiddling would never be touched with a ten foot pole in CANON because all of the normal developed world rightly sees that as the most disgusting, depraved, and evil thing in existence.
Again I'm not actually saying it matters with cartoon characters since they're not real, just that that's the main reason the show would never do it even if Bubblegum and Marceline would up both married to men as housewives with 12 kids and church every Sunday. It would never, ever be with Finn, ever.
Now the Flame Princess plot was bullshit. That was more about the "subverting expectations" thing. Also Huntress Wizard was the "growing up and mature" love interest for when Finn becomes an "adult" thematically, but they don't focus on it because "independent hero" shit. But none of that shit has anything to do with Bubbline. If it didn't exist, they'd just have not-Finn boyfriends or be celibate forever and focused on politics/hobbies but they were never in the cards for the simple reason Finn was a kid, they were young adults (actually centuries old too).