The Thompson was designed, and built during the era when all guns were built essentially the same way. Heavy steel actions, durable wood, made to last a lifetime. And military arms also had the need to be durable enough to be used as hand to hand weapons as well.
Development of the SMG, as far as what works well, better, best, ergonomics and construction methods was slower than most other arms, because theses lessons are essentially, only learned during war. AND which lessons were most applicable changed during war, specifically WWII.
It was during WWII that the world's militaries finally showed that they recognized that a gun that was built ruggedly enough to survive generations of soldiers was simply a waste of money in modern warfare.
Cheap, easily made (including in terms of manufacturing time) and able to work well enough, and last long enough was better for the military than an expensive gun made in the traditional manner used for making sporting arms.
Stamped guns, with a minimum of machined parts worked about as well in combat as the Tommygun, and if they didn't survive quite as long as more robust (and heavier) designs, they lasted long enough, and the low cost and ease of manufacture made them easier to rapidly replace when they didn't survive combat.
Newer designs are more useful, and easier to use. That is obvious. After all, if you don't learn from experience, and produce something better why would anyone buy it?
Today, the assault rifle fills most of what the SMG used to do for the military, and outside of certain very special situations, the day of the SMG is essentially over.
Obsolete, antiquated, and yet iconic, the Tommygun is like the Luger P.08, and the Single Action Army Colt revolver, one of the most recognized firearms ever, and while long past state of the art, still work as well as they ever did.
