I can't really agree. Though my experience is very different to yours by the sounds of it.

I use roulettes to get to level 61, then spam dungeons until 70.
I did the same in HW (roulettes to 51 then spammed dungeons until 60).

Further on this, you don't learn much at all for a lot of jobs until you hit the higher-60's anyway, as you're still locked behind a few key skills that come in the later levels that are pretty paramount to your rotation..
I will simply use DRG as an example here - you don't get Mirage Dive until Doma Castle, and Nastrond comes at 70. So the whole basis of the level 70 mechanics for DRG don't even start until 68/70... DRG isn't alone in this as well - even SAM gets Hagakure at 68, which is the backbone of a lot of your level 70 skill usage.

Most of the learning process from my experience is getting to 70 through dungeons, then spending a good while hitting a dummy while I figure out the rotation. This is not because "level equivalent content is scarce" but because core skills for each job come in the later levels so most of the leveling process from 1-65ish doesn't really matter to begin with.