I've said it many times before, but adding in the rapid-aging aspect to Finn's poison situation was a very smart writing decision due to the way that, like, instead of keeping the audience separated from the emotional state of the characters due to our knowledge of how Finn is 'supposed' to die…
It manages to give us a taste of the genuine dread and uncertainty that the characters are feeling.
And thus also makes us feel a very similar relief when his death is prevented this time.
Of course, another thing that I said a bunch of times is that, like, there aren't many characters confirmed dead in 'Together Again' that are still confirmed alive in the 'Fionna & Cake' Era, but one discrepancy that does make it seem unlikely for 'Together Again' to be the immediate sequel to this season is the status-quo of the Candy Kingdom…
But, up until the last episode I still had that little spark of worry, that some sort of Wacky Adventure Time Shenanigans are gonna make Peppermint Butler the new Princess of the Candy Kingdom before Finn dies…
So it also feels very appropriate that just as Finn's life is saved and we know for sure that he's not gonna die in 'Fionna & Cake', we get this reminder that the world in which Finn will die will come eventually, and the steps leading to it are already set into motion…
A reminder both that, no, the mental struggles he went through since Jake's death will not be singlehandedly cured by Huntress Wizard perfectly…
But that doesn't detract from the value of the healing that was done, that while the grief and the longing will not pass until 'Together Again', he's still doing so much better mentally than he was before.
And also just a reminder that Finn's eventual death is inevitable. He is still a mortal man.
And seeing how most of his loved ones are immortal and are definitely going to outlive him (except Simon), they will all have to face the actual grief of actually losing him eventually. When we've seen so many hints through this season that they will not handle it well.
But still, this little reminder of what's to come is not, like, ominous or sinister, especially considering the rather happy conclusion of 'Together Again'. I'd call it more… Bittersweet?