It wouldn't make you feel slighted. That was my point, which I may not have made very clearly. The reply I was making was trying to point out that adding a 3rd free retainer was not a negative to anyone - which is why I said I couldn't see a problem unless some people who pay for an additional retainer would somehow feel slighted that now everyone gets one - which is, honestly a ludicrous way of thinking. Of course as you say if you currently pay for 2 additional retainers you'd go from 4 to 5 if an additional free one were provided.
Since you both mention the amount of space required, let's throw soe numbers at it. Assume 5 million accounts with an average of 5 active characters with 3 retainers a piece, and more or less 200 inventory slots a piece (including shards, crystals, clusters and gil), that's 15 billion inventory slots. Let's assume 100 bytes per inventory slot, that makes 1.5 trillion bytes of storage. Now let's add in an average of 2 additional retainers (paid) per account (which is probably a very high estimate IMHO) and that number of slots becomes 25 billion, or about 2.5 trillion bytes.
Oh, but wait there are how many servers? 64, so break that down roughly equal over all of them that drops down to 39 million inventory slots per server (just for retainers) which multiplies up to about 400GB per server for retainer storage - assuming 5 million accounts with 5 active characters and 3 free retainers + an average of 2 paid ones per account...
Given that the cost of storage has essentially fallen through the floor, that's really not a massively significant amount of space per server.
Given that the number of active account and active characters per account are almost certainly lower than my wild ass assumptions, and the number of paid retainers on average per account are also almost certainly lower than 2 per account, I don't honestly see how providing a 3rd free retainer to players could be much of a problem to anyone. It's just database storage space, there is no other real implication involved. I don't know how much space they reserve for each inventory slot, 100bytes is probably a high estimate, and with compression the effective space required would be far less in their database, but this is kind of a worst case analysis, so....
When you aggregate all of this across the entire game it sounds preposterously big, but when you break it down per world, it falls into a far more manageable framework, and really becomes a much less impressive issue with the chief impact being the need for more database space. But given the huge amount of data in the databases already, adding an additional free retainer is like dropping a stone into a pond. You'll see a ripple and then it will be swallowed up by the pond.