As much as I dislike the AR type platform you are right. With one lower you can swap out uppers to many calibers. The AR type is a bit more ergonomic than what this Franken Thompson appears to be.
Of course you cannot use an ultra cool 50 or 100 shot Tommy gun drum in an AR...……..
There were some AR-15s put together that use Thompson stick mags. The base firearm was the Cav Arms AR one-piece body which had a magwell that was wide enough to accept Thompson mags, and IIRC, Hahn made the magwell adapter.
I've seen discussions about side feed, downward eject (Sten like) AR uppers for Thompson stick mags, but I don't recall any being made. And in a similar vein, I've seen discussions about MAC 10 and/or 11/9 uppers that would use drum as well as stick Thompson mags, but I don't recall any being made.