They can't force but they can't retract from the AppStore. You'd still be able to deliver outside the AppStore with a signed App on MacOS but with way less visibility.
Who's discovering DropBox from the App Store? Seems like they could drop that and be just fine. Much smaller devs have.
Who installs anything to a desktop computer from an app store instead of the developer's website?
I prefer installing things from App Store on my parents' computers cause it will handle automatic updates and provides certain guarantees (i.e. can rely on Apple not letting malware onto their App Store).