What if instead of a normal cash-shop, you have to "unlock" certain options?

[Seraphor]

To be fair, many games have something a bit like this, usually called a 'battle pass' and while sometimes it's a rip off, sometimes it's actually good value.
For example, a completely different type of game, but Dead By Daylight has 'The Rift' which has a premium version you can buy for 1000 Auric Cells (cash shop currency, about $10). You can upgrade to premium at any time even when you've already completed it, and if you complete the rift, you will end up gaining back 1000 Auric Cells in total, plus about $30 worth of cosmetics. If you play enough, it pays for itself and much more.

A similar implementation in FFXIV might be something like... a premium Mogtome event.
You get to purchase access to the premium Mogtome store for $10, which has things like Cruise Chaser for 400 tomes, or a Fantasia for 200 tomes. So you could grind out your 600 tomes first, then purchase premium, and get a $30 mount and a $10 Fantasia for just $10 in total.

I'm not suggesting they actually do this, because I'm not confident they'd actually price things in such a way that it was actually a substantial saving, but it's an example of how it could work out. Basically, it's not a totally bad idea, but... it's unlikely to be implemented in a way that's actually good.