Don't get me wrong. It's not that we don't need more inventory space. It's just that I think this is the worst possible solution, considering the options.
Why not make each retainer hold more items instead of adding a new one?
Why not make an inventory system like the one in GW2 where your crafting materials dont really occupy space and instead are all available at all times? God knows how seriously fulfilling it was to open that tab and see ALL your crafting items, and how many you had of each.
Bottom line, retainers are more annoying than they are helpful. it is super annoying navigating between one retainer and the other. And it is even more of a pain to have to look for items inside your retainer, which you may or may not have. It's like they are expanding on what is a really bad inventory system. i like designing a retainer, but this hassle aint worth it.