While I understand your want to use leves to gain the majority of your SP without leveling other classes, the crafting class SP needed to level is identical to that of battle classes. Until they put something into the games to leve-link or GA for crafting you will not gain the kind of SP that battle classes can gain. In order to make equipment for battle classes you will need to have crafting classes leveled. To level your crafting classes you will need materials. You can obtain those materials from the gathering classes and in some cases (Leatherworking) from battle classes.
Not wanting to level another class to complete your task is perfectly fine but you will no doubt take longer to accomplish your overall goal. Adding more leves only serves lop side things even further against your goal meaning that you will still level slower than others who have leveled their gathering classes anyway. I see you point that there should be more diversity, but my point is there are other ways to accomplish this already without those leves.
As I already said, not wanting to level another class is your choice so by its very nature a limitation you choose to place on yourself. Your personal choice to do something or not do something does not necessitate a change in the game. You do not HAVE to level the gathering classes...your penalty is that your crafting classes will level slower. Just like a battle class does not have to level their crafting classes...their penalty is having to wait for someone who can fix their gear to be around/willing to fix their gear or get a reduced repair from an NPC at a ridiculous cost and they will have to hunt down the gear they want through NM's and what happens to be in the market wards for them at any given time. It was the same way in FFXI. You never had to complete your SJ quest if you chose not to...your penalty for that decision was less party invites. Crafting was no different in FFXI either...you did not have to level a particular craft...your penalty was that anything you needed from that craft in order level your chosen craft you would need to buy on the AH. This system of dependency is just more apparent and frequent in this game.
Like I said before in my previous post, these are all just suggestions. You are already aware of how you CAN do it, you are just making the choice not to. Please realize though that these problems you are encountering are because the devs have to balance things out for those who are choosing to do it the way I am describing.