Purchase of spells seems like a bad idea to me, though some form of crafting would certainly fit XIVs feel, if be very difficult to implement for the sheer number of spells they would need to produce for it to be a similar crafting tree to the others.

I would assume that the intention at the start was to add new spells and equipment in the first expansion (assuming one was/is planned) but that the expansions has been put on indefinite hold when they fixed what is already there. In think that and raising the level cap to 55 or so was likely what they had planned at the beginning.

I like the current system though, I like that while there may only be a dozen or so abilities per class you can swap a lot in-between them, ok so adding ARC techniques to your THM ability set won't work but it is still one of the most flexible systems around and I would rather they add only another half dozen spells or so and have them keep the current flexibly than gain 300 new spells that have to be class locked for balance concerns.

Perhaps purchasable single use scrolls with a greater variety of effects than the default spells may be the kind of thing you want them to add?