I don't understand the difference between in-ear monitors and hearing aids well enough to give a reasoned counter-argument to that.

However, Costco does sell hearing aids for 400-1000 USD while audiologists sell mostly comparable equipment for 3000-10000 USD. So there is definitely pricing power in the current setup.

Most people in the industry also expect lower price devices to come to the market when the bar to entry becomes lower and manufacturers can start experimenting with new size/form factor/battery/functionality combinations. For example Bose Hearphones were selling at 500 USD as a low-volume product without much competition.