So, when Finn and Jake make it to the time room they see Prismo granting the Lich's wish. Prismo explains that when the Lich wished for the extinction of all life he changed his own timeline, but Finn and Jake are safe because they are in the time room. At this point Jake is ready to wish that all life wasn't extinct, effectively negating the Lich's wish, before Finn stops him.

But, what would happen if Jake actually made that wish? He would be sent to a timeline where all life wasn't extinct, and the Lich would still be in a timeline where all life was extinct. Meanwhile Finn would be left with Prismo in the time room awaiting his wish. That, honestly, doesn't seem too bad.

But, in Jake's new reality, would there already be a Finn like there was another version of Jake in Finn's farm-world reality? Would there be a new Lich or would he still be in his life-extinct reality?

I suppose what I am trying to illustrate is that if the farm-world reality, where Finn is wearing the crown and fighting Lich-Jake, is still ongoing like many people claim, then Prismo's wishes seem to propagate new timelines and create a lot of confusion. Maybe this is one of those things that you shouldn't over think.