I've never heard anyone complain about the 32-40. Maybe you are thinking of the 32-20? Admittedly the 32-20 is not a powerhouse, but for most of its existence it has been considered a perfectly adequate deer cartridge. Today it is often loaded to around 30 carbine and 327 federal levels, and they work fine for deer. Again at that point we are talking supersonic, but also not very heavy bullets. A 300 BO at 1050 fps and a 200+ grain bullet is a fine deer cartridge... within its limitations. Provided you get good expansion, it does not lack wound channel or penetration. It does not have either in excess, but it does work.
Consider this, today airgun hunting is growing in popularity quite fast due to the bigger and more powerful airguns now being easy to find and not overly expensive. They share a lot in common with the subsonic firearms, both are liked for the lack of noise, and general ease in shooting. Quite thorough testing has been done with airguns, on whitetails specifically. A 150 grain bullet at 800 fps is considered adequate, about like a 38 special handgun. I will agree that when it comes to subsonics I would think bigger is a much better choice. I would think a 450 bushmaster, or even a 50 beowulf would be ideal for big game hunting in the AR15 subsonic.
America loves its 30 calibers though. And yes, people really are buying 300 BO subsonic and are really hunting with them with good effect.