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As for the question of why, it’s certainly within the realm of possibility that they just don’t feel like maintaining it anymore.
I think this may be true. After my last Mac upgrade, the app did not work with either of my phones. It looked like just a timing issue, because the files would eventually appear but the file transfer app had given up and wouldn’t let you do anything but quit. Probably a fairly minor bug that for whatever reason they just won’t fix.