The Thompson was substantially heavier to lug around than a carbine, and even out of a longer barrel, the .45ACP's trajectory is nowhere near as flat as .30 Carbine. Initial runs of Thompsons (M1928A1) were very expensive until they streamlined the production process with the M1 and M1A1 variants. What you're suggesting (i.e. a .45 ACP with the weight and production cost advantages of a carbine) basically took shape in the M50 and M55 Reising submachine guns, and the M3 Grease gun. Granted there was a good bit of overlap, but not enough, IMHO, to dispute that they did not fill distinct niches.
I only meant that logistically ammunition production and distribution could have been simplified by eliminating a unique round in the .30 carbine. The cartridge pressure is not that great, so all the M1 carbine would need is a larger barrel which would add minimal weight. I already addresed the effective range of the .30 carbine. The weapon was intended as a last-ditch defensive weapon for non-combitants that was something more than a sidearm. The longer barrel and sight radius of a .45 carbine would have made it suitable to the role intended for the M1 Carbine in .30 carbine, and would reduce the logistical problems. No need to make a unique cartridge. One less chance that your supply depot will have a bunch of .45 for Thompsons but no .30 carbine for the M1.
I think it was a win-win. I don’t see why they went with a unique, low powered cartridge that was no more effective than a pistol round. Why not chamber in something already produced in quanity and simplify your supply problems.
Again, I’m just not seeing a downside if the .30 carbine didn’t exist and if M1 carbines came in .45 ACP. If this weapon was intended for offense, then yes, and even then the aenemic choice of round is not suitable. For a close contact last-ditch defensive weapon for truck drivers and cooks, I just don’t see why .30 carbine is any more effectiev in that role than .45 ACP would be.