The game wasn't so much "designed to support hybrid classes" though. The Holy Trinity works. I don't think anyone can really refute that. So many games use it because it works. The possibility of hybrid classes in the future is simply the reason they chose the arbitrary number of 8 people in a Full Party. It obviously works just as well with four DPS instead, so if at any point they've decided that hybrid or support classes simply aren't going to work out within FFXIV's setup, I have no doubt that they'll simply scrap the idea and make RDM, BLU, or whatever other classes they decide to add fit into one of the existing roles. Like I said, we obviously aren't getting it for Heavensward so if it's still happening, probably the soonest we can expect it would be the new expansion. So another year, possibly two?
My previous game has a lot of hybrid classes. You name it, there's a hybrid for it. The problem you run into is that unless the classes are exceptionally well designed, you end up with horrible game balance issues. If they're underpowered, they end up being underplayed. People don't want them in their parties, so they get kicked in favor of stronger classes. Nobody wants to be kicked from a party simply due to their class choice. If they're overpowered, then that's all you see people playing everywhere you turn. Parties with max numbers of that class become the norm and all others end up being kicked in favor of stacking up that one class. You end up either overhauling classes one by one (which has problems of its own) or overhauling your entire class system in a quest for game balance. It all turns the game into a huge mess, which can end up turning players away when new content is introduced that suddenly makes their chosen class obsolete.
SE already has a well-balanced game here and I can't blame them for being so careful.