I completely support Stefan's response - if you decide to pay for Paperpile, you should do this for the features it already has, not for the features it might have in the future.
As a workaround, you could consider using Paperpile in combination with Mendeley. For me, the main value of Paperpile is integration with Google Docs. But Mendeley is free and more powerful in other areas: it has full text search, and it has mobile apps. If you synchronize your Paperpile Google folder that has PDFs on your computer, and then make Mendeley watch that folder, you can automatically update Mendeley library with the PDFs imported by Paperpile.
One wrinkle in this setup is that annotations are not interoperable between Paperpile and Mendeley, so you would need to choose one that you will use for annotations.
I do hope one day I will be able to use just one reference manager for all of my needs!