I guess best option is to implement AH but leave MW as well. Give it 1 month and we will see which option players prefer. And I dont know a reason why both AH and MW cant exist simultaneously. Im pretty sure tho, if there was AH and MW - one MW row would be enough to hold all the retainers.

About 7 days (*) limit in AH against no time limit on retainers... what kind of argument is that? The question is if you want AH, not if you want AH with time limit.
If you ask me if I want FFXI AH in FFXIV - I dont! Well, even that would be better than Market Wards but it is definitely not what I expect from next generation MMO. I expect a tool much much better than AH in FFXI. MW in FFXIV is much much worse so far.

(*) Im pretty sure in FFXI its 9 days but thats not the point.