expansions could fix that,the reality is there is no reason to over-complicate a game when it first starts.

Take a look at XI for example, things like BLU, SMN, PUP, BST, DNC were not implemented til much much later on. You started off simply with basic class warrior, thief, rdm, blm, whm and monk.

Warrior auto attacks, provokes and uses WS when they can, rdm/blm/whm used spells, thief auto-attack and ws til they got a bit more variety at 15 with back attacks and then monk pretty much auto attack and ws.

Come the expansion, they have an opportunity to expand the "roles" and add more variety to the game without making the starting content harder to get into.

You can see it as, the content we started off with is for ease of access, to get into the game and lower the wait time, come the expansion maybe they'll add two roles, support and enfeeble, and add in jobs to allow DD roles to fit into those two roles if needed.

Then the dungeons and other end game content for those expansions would use those two roles as a way to increase the strategy of combat but of course it would be introduce in a way that wouldn't overwhelm new players just getting into the game. They could start off getting to know the basic roles, going through coil and CT and when they geared up enough they could move onto the expansion content and be given an expanded experience by being introduced to two new roles in party mechanics.