There's a lot of things that could help their inventory issues: A glamour log, a tackle box, moving all the currency-like trading tokens to the currency tab, etc. They should also have expanded the armoury chest when they added additional jobs.

Shortly before 3.0 came out, Yoshi-P acknowledged that they were aware inventory was becoming a major issue and needed to be fixed, but that since it was going to take them a while to fix it, they'd offer additional retainers in the interim. Then it turned out that they didn't offer additional retainers with the game (or even with purchase of the expansion), but only the option to purchase more extra retainers than we previously could. That's kind of a low blow when it's a substitute for fixing acknowledged issues in the game. We should have gotten at least the 3rd and maybe the 4th retainer as standard (with extra purchases for the 5th and 6th), or, as you said, additional space with each existing retainer.

And then they should still go back to working on the more significant fixes mentioned above (glamour, tackle, currency, etc.) that make inventory itself more functional.