That
might be true from a technical limitation standpoint, but they can do anything they want to do, they just have to want to do it, and it has to make sense for their bottom line.
The game already tracks which Trusts you have, and even refreshes dynamically in Gaol with a "no repeat trust" limitation by removing that trust from your view temporarily when you've summoned them. They probably could create an NPC (or use the ones that give you new trusts via Ciphers) that allows you to add/remove trusts from your summonable list. It would be like Blue Magic: a list of all available, and a list of what's currently equipped.
They just haven't figured out how to monetize it yet.
"We
can't add anymore inventory, there's no room" (people been asking for years)
"OH WAIT we can do it, you have to pay for it. By the way, you need 3 unique jobs to enter Gaol, so gear tf up!"
"We
can't do race change" (people been asking for years)
"OH WAIT, we can do it NOW, you have to pay for it, and here's an event that gives race-specific rewards"
"We
can't release Cornelia/Matsui-P, we don't have any more IDs"
"Still thinking of a way to make you pay for her, but haven't figured it out yet"