Pay to win is anything that you can spend real money on that gives you some sort of advantage. More inventory space IS an advantage. Yes, it is also a convenience, but that does not change it from being pay to win. And that ignores the other advantages that more retainers bring - more Ventures, for more stuff gathered, either targeted stuff or from some random list; and the ability to sell more stuff on the marketboard, resulting in a potentially greater inflow of gil.

Saying that retainers are not pay to win ignores the aspects that make them pay to win.

Heck, even cosmetic junk is pay to win, if you are discussing Eorzea's Next Top Model! Pay to win the fashion contest, at any rate.