I have shot both a .45 MP5 and a thompson. I will tell you the MP5 is easier to shoot well and it handles much better.
The Thompson was an idea based on a concept that some would say started with the civil war. Trench warfare was a horrible thing. The generals did not learn a lot, but they did learn that although getting a toe hold in an opponents trench system was difficult, it was far easier than trying to hold on that section of trenches when the opponent could fit you from both sides and from the rear. The load on sunday and shoot all week Henry rifle was the initiator of some new thoughts, A full sized full auto weapon was unwieldy, and if it carried rifle sized ammunition it was so hard to fire accurately and so large as to be really impossible to use in assaults. The SMG of the first generation were really rifle shaped guns that fired pistol rounds, holding a simple switchable magazine that allowed for a useful amount of fire. Look at a Lancaster, a Bergman, a Reising, even the PPsh 41. wooden stocked, magazine positioned out in front, machined parts,
Then look at the second generation of SMGs, the MP40 the Sten and the M3, These were all weapons made more for manufacturing simplicty than fineness of gunsmithing. GM was making Stens for roughly nine dollars a gun by late 44. The M3 was not much more. stampings, reduction of features, (the M3's use of a cocking hole in the bolt.) All were endeavors in getting the most guns out the door the fastest. Most of WW2 combat was taking place in close ranges. Urban fighting in Europe, Jungle campaigns in the pacific, forest and mountain fighting in the balkans and massed attacks of soldiers against fixed positions with armed mounted infantry clinging to tanks and firing madly against infantry positions all were tactics that pushed the SMG to the forefront.
After the war, the role of SMGs started to fade as the move to air support and integrated combat operations tended to move the contact ranges back out. Interest was developed earlier in a DP gun, small enough to function as the SMG in the MUOT and yet capable of fighting as a battle rifle when the terrain allowed. The AK and the AR are both products of this thinking.
They have pursued the same objective via different routes.
I will not make any judgements on that discussion, but to say that the SMG is dead is also false. I have talked to two gentlemen who of late were in Afghanistan working there for our Uncle. Both chose to carry MP5's as their primary weapon. Both were not line troops, but rather specialists who were operating in the realm of nation building and were supported by others who had longer range weapons and rarely operated in contested areas without some sort of air power available. However in their role, the MP5 provided enough firepower to do the job and they preferred them to M4's in the rabbit warren type compounds they were often in. Both said the 9mm 124 grainers were much more effective against people than the M4 at inside the ropes ranges. They both related to me seeing several BG's get 4 or 5 5.56's in the chest and not go down, they never saw that with 4 or 5 nines. They did see several guys hit with both who took one round and were martyrs, but the claimed more seemed to take 5.56 hits and keep fighting for a while, while several hits from the 9 seemed to anchor them. Both said that in certain areas of OPs, 124 Rangers were allowed and they say that drastically reduced the operating window of the BG's.
The Thompson is just cool in its own right. We all like 66 GTO's or 67 roadrunners. Big strong and powerful, they are none the less, outdated.
No AC, bad suspensions, horrible build quality, cheesy interiors, all pale in the light of even a modern Honda or Toyota on road manners, comfort and utility. A 66 GTO with a tri power 389 is a fun beast to play with, but I would say most of us would really prefer to leave it as a toy and keep a Accord for a daily driver. (I would kill for a 68 RR with a $741 option 426 Hemi)