To be honest, I once tried to work on my RPG plugin project to make it platform agnostic such that it would work on both Bukkit and SpongeAPI, but alas, I’d be reinventing the wheel with the strides SpongeAPI has made.
To be honest, from my personal point of view, I once worked on plugin commissions for Bukkit and as soon as I started questioning the amount of bloated logic to perform some basic operations with Bukkit/Spigot’s API, I stopped doing plugin commissions.
In general, I wouldn’t want to contemplate having a SpongeAPI implementation on CraftBukkit/Spigot because of the amount of differences the two API’s have, it would be incredibly difficult to try to implement, unless you’re creating a SpongeAPI implementation directly on top of Spigot.
Again, this is all my personal opinion.