It's not as simple as you put it. What it would do is allow people the AH system IF THEY WANT IT, but put it at a price for the convenience.
For those who enjoy the shopping experience (and some people DO enjoy it), you can still wander around.
If you ONLY used the instant buy system, then, yes, you are essentially eliminating the retainers. However, unless the overwhelming majority of the population went to using the "buy it now for a markup" option, there is still an argument for a MW system.
--edited for spelling...I cannot type sometimes--