I think that you are underestimating the use of the Mac App store. Many applications are only available in the app store, just to give some examples of popular Mac Apps: Pixelmator, Affinity Designer, OneDrive, Pages, Numbers, Keynote, iMovie, Garage Band, Tweetbot, and Airmail. Then there are many Apps that can be purchased both through the Mac App Store or from the vendor, such as Omni{Graffle,Focus,Outliner}, Fantastical, 1Password, Photoshop Elements.

The majority of apps that I install come from the App Store.

Moreover, some vendors also sandbox non-app store apps. E.g. Chrome tabs, Photos, or Safari tabs.