I agree that completely unfettered Mail access would give RMT too much power. (Not only for moving items/gil around, but who wants their mailbox filled with RMT spam?) But we're not asking for unfettered Mail access to everybody, just for access to our own alts on the same account. Those are characters with far closer ties to our current character than anyone on out Friends list has. Moving gil or items around between unrelated characters is already allowed, just by Friending them. That's the part that RMT would use it for. So they're allowing the part that RMT would use while blocking the part that they wouldn't, but saying it's because of RMT.
SE just isn't making any sense on that. It's so nonsensical that I'm about convinced that in that Letter from the Producer Q&A quoted in the OP they must have misunderstood the question. It really sounds like they answered why we can't freely send mail to whomever we want (maybe thinking of alts on separate accounts) rather than why we can't send it to alts on the same account.
If you have to pay more or pay longer and the ONLY difference in what you get for that extra money is more alts, then I'd say we're definitely paying for extra alts. It's just that it's a single amount for alts 2 through 8 rather than paying one by one.
Fantasia is already bound so that it cannot be transferred through other means (like FC chest or trading through a friend). That would work for mail as well.
Yes, provided your friend is always online and never busy doing other stuff, or that you're in no hurry to get the items you transfer if they're not. And of course when you decide a friend is trustworthy enough, you'd better be right about that.
And yes, this is indeed one of the workarounds players use to make up for the lack of this basic feature, but it's enough worse from the requested access that it doesn't really make for much of a counterargument.
And this is the problem with the other big workaround. If you want to use a private (or really small) FC so that you can transfer stuff, you have to give up on all the other purposes of having an FC.
I don't think it has anything to do with selling retainers. This was frequently (sometimes rather vehemently) requested back when ARR first went live, and there were no retainer sales at that point, nor for a long time after. They didn't exist when that Letter quoted in the OP was written, for instance.
If they ever do, it had better be a separate feature not tied to those alts' own extra retainers. Right now if you have two or more alts and purchase an extra (3rd) retainer, all of your alts get a 3rd retainer. If you're genuinely playing each of them, they'll each have their own accumulation of items and will each need their own storage (and sell slots). Replacing that with a single 3rd retainer they all access would give you easy transfer, but at the cost of losing the ability to store and sell items for each character.
I'd like to see something like what LOTRO has, with Shared Storage that lets you share items with your alts as one feature and an upgrade-able Vault (per-character storage) as a separate feature, though they're accessed from the same place. Our Retainers would be the per-character version. Let us have another shared chest or other type of NPC lackey to act as the account wide shared version of retainers.