Mojave App Store Only Shows "Updates" Tab
Never mind, just figured out an easier way to resolve this:


Spotlight > System Information.
In the lefthand column, locate Software > Applications.
Find "App Store" in the list. If there are two versions, click the older one and identify its location.
Navigate to that location in Finder and confirm (via right click > Get Info) that it's the older version. Delete it.


Voila!