The Sten MkII is not "simplicity", rather it is an example of extremely intelligent design.
If you were to class subguns into generations, one could argue that a STEN MkI is a Gen 1.5 subgun, but a MkII has leapt the MP40 and PPSh into a Gen 2.5.
In reality a MkIII Sten is a MkI* converted to modern production methods, truely analagous to the relationship between an MP38 and an MP40