There seem to be two separate issues with regard the salt mixes: a) does it actually go bad say due to organics expiring b) how much and at what rate does the salt change?
I mention B, because it seems the majority of people already use salt that doesn't exactly match their desired tank params for normal WC (Alk, Ca, etc). Some people adjust the new salt mix to better match their tank with supplements and others adjust their tank dosing regiments to account for the change. If it's the latter, using a AWC of a small amount seems to make it easier to handle those differences. The real question is how much the salt changes over time. A max Ca change of < 5% over 30 days doesn't seem like big deal when on day 30 you're replacing 1% of the water in your tank with salt that's less than 5% different than day 0.
The viability of AWC doesn't seem in question. Lots of people are having success with that. The less the salt changes, the easier keeping the tank balanced.