You need to remember that we as players don't see what may or may not be behind the scenes. If I were a game developer and would introduce such a system where you can buy retainers for real money, I'd keep that in mind when designing future content and made sure to have many many more items and materials and items (even if the game can manage just fine with the current materials which some of them don't even serve for anything useful), in order to get the players to need more space.
For example if some kind of weapon enhacment item come out, I'd make sure to have different item to serve that for each weapon type, making players need to occupy more inventory space, instead of making it more convient and having one type of item for all weapons.
All the players wouldn't even suspect that's what made me change the design of the game and the reason I did it.