As one of the former Papers for Mac devs (before it was bought by Readcube), I can feel the pain.
Like others in this thread, I’d like to recommend Bookends. While Bookends may partly work different compared to Papers, it’s well maintained, continuously developed and has world-class support. And as the case with Papers shows, these things may be far more important long-term than any differences in features or usability.
Like Papers, Bookends 13.2 and above is nicely scriptable. This is a big plus IMO since it allows for customized workflows and also avoids lock-in.
As a case in point, for anyone considering moving from Papers to Bookends, I’ve written a “Papers to Bookends” script which exports all publications selected in your Papers 3 library to Bookends:
https://www.sonnysoftware.com/phpBB3/viewtopic.php?f=6&t=4360
The script will also transfer the publication’s primary PDF (if there’s any) and can also transfer the publication’s rating, color label, flagged status, language, edition, citekey, and “papers://…” link.
The PDF file’s annotations can be also transferred if you follow the steps given in the “Known Issues” section of the README.
For macOS 10.14 (Mojave) and above, you may need to copy the script’s code into a new ScriptEditor.app window and run it from there. Also note that, for larger libraries, the script may take some time until finished. But the script reports its progress, and it seems to work for many users. So this should ease the migration path from Papers to Bookends.
Hope this helps.