You are completely overstating the advantages of a hold open. Unless you physically look at action you can't tell.
In WWII the carbine was a excellent choice, your complaining about only having 15rds of semi-auto in a 6lb package, meanwhile the people they are fighting are armed with a 5rd bolt action weighing at least 8lbs. Literally enough firepower to take out a whole section without even a reload if everything lined up perfectly.
In Korea it was still one hit to get them down, just like the M1Garand and in both cases it depends on where you hit the person. The only difference being many people were missing with M2s because they were choosing to use full auto instead of semi-auto and effectively aimed shots.
The M1 Carbine saw effective use in many countries for many years, something like that doesn't stay in service that long unless it was a good choice. It was extremely effective for the intended role as well, as I doubt you would be better off with a 1911 pistol in any combat situation.