There are a lot of AK74 owners out there and with Smith making a complete rifle, the MP15R and the avail upper only along with uppers in 5.45 made by other manufactures it's only a matter of time for US ammo manufactures to begin producing the round. Add the fact that the run on guns has driven up the number of 5.45 rifles in the hands of shooters helps to push demand up for the round and ammo makers know this. With enough demand do you really think that one or several of the big ammo companies is going to ignore demand and the ability to make more money by offering the round? Another fact to consider is (and I would bet money on this) Smith & Wesson is sharing sales figures to the big ammo makers showing how many of their 5.45 rifles have been manufactured and sold. It's even possible that Smith was in talks with the ammo manufactures prior to the rifle being on the market basically saying...."hey..were going to be making an AR in this caliber....there's going to be a market now for US manufactured 5.45 because as you know...AR shooters can be picking about what ammo they shoot out of their ARs". I'd almost venture to guess that companies like Savage will begin producing a bolt rifle in 5.45 in the very near future as well.
To summarize, I don't think this caliber will fade into obscurity with the number of rifles is the hands of American shooters.