The thing is, FF14 is still growing in sub numbers with the current inventory system. To shareholders, CEOs, and people who prioritize profit, you think they care about how a revamped database -could- help the game when the current method is working just fine? (Especially when the current method allows even greater profits by making people pay for extra retainers instead of getting space...y'know, for free?)
For the amount of work it'd re-take to redo a lot of the game's most core foundations, it's likely entire patch cycles of dev time would have to be gutted. Try explaining 'yeah, we have to cease all content production for 6-9months which is the stuff that keeps our playerbase subbed and staying in-game to change our code foundation to implement features that technically have zero tangible positive effects on increasing player counts and any benefits to sub counts can be only speculative' to the CEOs & shareholders of Square and see how quickly they look at you like you've gone insane. To the people actually giving Yoshi-P and his team the funding for FF14, it is a side issue, since it would interfere with the content cycle which is what actually makes FF14 its profits. Look at how people lost their minds over the long gaps between 5.2 <-> 5.3 & 5.5 <-> EW to see how well that'd fly over if you told the playerbase they're in for another ultra long break.
But that being said, I honestly don't think it's an issue related to their item databasing. I think it's an issue of server I/O load limitations.
Yoshi-P has constantly alluded across many Q&As to the fact that any planned increase in inventory has to be done with extreme planning due to the fact that with how they've designed their Server<->Client interactions, the inventory chest & armory chest are always loaded so that they can be backed up every few milliseconds. As a result, even a single additional item slot being added increases the server I/O load by the size of that slot * the number of currently online players on every read & write pass to the server. For an example, if we pretend that each item slot is 1kb in size (which its likely larger) and pretend there's a million players online, that 1kb of extra inventory space means the server is now backing up around an additional GB of data every few milliseconds, adding up to TBs over the course of an hour. And that's if its just a kb to start with.
Taking a look at the Eorzea Database, there is roughly 30k pieces of equipment when you filter out non-equipment items and leave only actual equipment. If each item only needs a single bit in the glamour log to determine if you've unlocked it or not, it'd need about ~4kb total. If we pretend FF14 has a million concurrently online users at the same time, that easily translates into well over 10TB of data passage every hour.
If the datacenter Square is hosting one of their DC's hardware has a special contract that says the FF14 hardware cannot exceed a certain amount of data per month, the reason behind their extreme hesitancy to increase inventory ever becomes much more clear. Especially if the contract cannot be changed for higher I/O throughput because of financial decisions by higher ups at Square. It's not an issue they can or not; even with their current code foundation, they can always find a way to create some hacky extra way to add in something. There's clearly some unseen reasoning, likely something beyond Yoshi-P's control that's blocking them.