Minerva was the first one to come up with the idea of digitizing Finn, but she did internalize her lessons from "The Light Cloud", now she would just like Finn to enjoy all of the Meatspace things that he loves and she cannot have anymore…
But it's Princess Bubblegum who pushes her to go through with it, and honestly… it kinda seems like it was a plan she had in her backburner for quite a while? Like, calling up Minerva was a "welp, I guess we have other choice", not a "Wait! I have a NEW idea!"…
And she sure pulled out that Indestructible Robot Body pretty quickly…
I think Princess Bubblegum has been planning this out for a while now, she just hoped it'll be for when Finn is at risk of dying from (actual) old age and not right now. Immortality almost seems like the default to her since this is how her girlfriend and many of her other loved ones operate, and… in general it has always been hard for her to accept endings.
But more than anything, Finn is a really important friend to her, and knowing she was always going to outlive him… instead of handling this preemptive grief, she was already making backup plans to keep Finn around forever.
That might also be why she was so insistent on the idea of pumping him full of Super-Science-Botox during 'The Bird in the Cloak'. She says it's for the greater good and keeping people's 'morale' up, but I think the real morale she was trying to keep up was her own. Because the wrinkles were a reminder of Finn's real inevitable mortality.
And Bonnie might not be alone in this instinct, Marceline also gave into the urge to try and Vamp Finn very very easily…
It's possible that she's also been considering how she'd keep him immortal, in the only way she knows how.
For as much as we've been talking about how terribly Finn has been handling Jake's death, I think a lot of Finn's friends are not doing so great handling their own concept of Grief either…
Which feels very appropriate for this Season's themes for how we are all connected and all reflect each other. Finn feels totally alone in his pain, but so many of his friends actually have very similar problems deep down…
