It‘s also worth noting that their upcoming "desktop“ version really seems to be a rewrite of their ReadCube product, based on cross-platform web technologies and styled to look like a Papers 3 successor. Depending on implementation, the user experience may differ quite a bit from a native Mac application where it’s easier to closely follow system conventions and to offer good system integration (such as scripting support).
In another thread, I‘ve mentioned a script that I‘ve written to help users migrate from Papers 3 to Bookends.