How do I download an iOS App (IPA) file to my Mac after iTunes 12.7 update?
This isn’t exactly what you want, but just to add this option too...

If you use the "Apple Configurator 2" app, you can still download and update apps from multiple Apple IDs - as you also could with iTunes.


Connect the device with USB
Sign in under Account:





Choose Update in the menu:




A more detailed description can be found here: http://krypted.com/apple-configurator/upgrade-devices-and-apps-using-apple-configurator-2/

The Apps are downloaded to:

~/Library/Group\ Containers/K36BKF7T3D.group.com.apple.configurator/Library/Caches/Assets/TemporaryItems/MobileApps/