You say that the Thompson is better than the mp 40, but you have to take in reality.
I don't know why they didn't use the grease gun model over the Thompson, it was not only easier to handle, but it was significantly lighter. It weighed 10.8 pounds which may seem light but gets very heavy when you become fatigued, the grease gun was only about 2.4 pounds lighter but that made all the difference on the battle field. The MP 40 has a significantly lower recoil (in real life anyways) and is much lighter than the Thompson, not only that but it was cheaper to produce.
In real life, I think the MP40 was much better than the Thompson.