I'm not having a problem with inventory space, but I haven't reached a high enough level on enough classes to pose any sort of problem yet.
But shouldn't the solution be some sort of storage chest if the goal is just additional storage space?
Retainers bring much more to the table than just inventory space. The retainer ventures give items and the limit on how many items you can sell helps control the market (you have to be selective on what you want to sell at some point).
Increasing the amount of items people can sell, and the number of items retrieved by retainers by 50% for everyone is bound to throw some problems. If anything, just add a few more tabs of inventory space to each retainer. But until then, either pay for more retainers, or be more strict with your inventory management.