I think you're just not giving people enough credit on both sides. The players are smart, so are the dev team. They know the score and what's being said. The dev team isn't so stupid to think that a Marketplace that acts as an AH, just isn't called one, won't be good enough for people, and people similarly would be happy given all the standard options asked for.
I'm sorry, I just don't believe it would be some crazy financial drain, nor do I think that's a reasonable line of thought. I've seen smaller companies make bigger changes without a problem, and removing Retainers from the equation is a relatively small issue.
But it is unarguably detrimental to it, and that's the point. Unlike Expeditionary force content, which I could avoid and still function in-game, Retainers are an unavoidable - and entirely unintuitive, unnecessary - aspect of buying and selling. The process could be accomplished without them.If they're unnecessary, they won't be used. Why is that a good reason to take it away? What does it improve?
Let's say for example the Expeditionary force content in XI. It was a useless feature that no one ever did, but why would they have taken it away? What's the logic in that? It's not detrimental to the game, it's just there. Does it annoy you or something?
There is no reason to take anything away from this game as long as it's not detrimental to it. That is a waste of resources in the best sense of the word.
As for benefits, numerous ones have been mentioned. It would be quicker buying and selling, it would reduce server load, particularly if population went back up, it would be easier to manage sales, it would be a lot less time consuming, it would be less longterm maintenance SE would need to focus on, they wouldn't crash and kill sales, they wouldn't be yet another bloated menu system to deal with, it would make inventory management more streamlined, no more having to stop selling to summon a retainer to manage stuff.
I'm not sure why you keep saying there are no benefits, when many people have posted potential gains - none of which you have argued, even.
Simply relegating Retainers to act as mules would have numerous benefits to the banking, economic, server, inventory and other issues, and it really shouldn't be particularly hard to do.