Most content from now on will be added to high and mid levels. That's how the norm goes in MMORPGs, with dungeon and such. I seriously doubt, as the game progresses, that people at mid to high levels will find themselves with nothing to do. On the other hand, I find that the low levels are rather empty, and unless they add dedicated content (which I find doubtful) they may remain that way.
Remember that in a MMORPG the first impression is *very* important. At the moment the low level first impression of FFXIV is a tad weak, IMHO.
There are main scenario quests for every 5 ranks from 1 to at least 20. That's already too much at least for 1-10 since you level so damn fast. There are also a good bit of quests to do for those ranks as well. There is simply no reason to add anything else for pre-20. And like in my example, there are three class quests for ranks 20 to 30. That is plenty, and more will be implemented with the sidequests in patches.

Actually, none of the content added since December has been for the higher level people. The NM's are for ranks 25 to 35. The quests range from ranks 5 to 30 or so. You have no basis for thinking that they will ignore the lower ranks (especially when they haven't even implemented the tutorial quests yet, who knows how good those are and for what level range).

You are making up excuses for your idea to be viable, but there is nothing that suggests that SE is going to ignore lower levels in the future. With leves, main scenario quests, tutorials and sidequests newbs will have a busy time until rank 20 and beyond where the mid-ranks begin and the class quests among other features are presented to the player.

It's not a matter of rushing. The kind of SP rewards i hypotesized would speed up leveling very, very slightly. Besides, questing takes time (lotsa running, at the moment). It's just a matter of balancing time spent with rewards.
You can't talk about balance before we see where the game is headed. You don't know how valuable the gil and other rewards will be when the time comes. Your thought process is way too narrow and focused in the present, when the present is irrelevant.