DragThing is written using the 32-bit Carbon APIs that Apple have announced they will remove in the next major update of macOS after 10.14 Mojave, most likely in September 2019.
64-bit support would require completely rewriting DragThing from the ground up, a process which would take us six months to a year to complete, with no guarantees we could re-implement all the existing functionality.
Unfortunately, we do not believe there is enough of a market out there for a new version, such that it would be financially viable for us to do so. Almost all of our income over the last ten years has come from PCalc, and time spent on a new version of DragThing would be time we couldn't spend on improving PCalc.
While we have not yet made a final decision, we do not feel comfortable selling an app with an uncertain future, so DragThing is no longer for sale. It should continue to function on 10.14 Mojave for now.
Existing users can still download a copy here.