You learn what you take from it. I've played with people that have multiple 60s and still can't grasp their basic job mechanics (SMNs not building aethertrail, DRGs not upkeeping BotD). For others who read their skills as soon as they get it, can generally grasp it as soon as they learn it by piecing it together (such as DRGs getting each ability post-50, likewise with BLM). The game in general is so undertuned that poeple who don't know their basic job mechanics are the dime-by-a-dozen, and I hardly feel that getting a free boost to 50 would exasperate the issue.
This is hardly different from someone in EX that still doesn't grasp positional or buffs. The matter of fact is, all of this can be alleviated with a tutorial of sorts. The way WoW goes about it is that it throws you into a introduction zone where it introduces abilties specific to your class 1-by-1 and straight up tells you how to use it. By that time if you can't grasp the fundamentals of that, I doubt the leveling process of 50-60 or 60-70 would be of any help. We're not talking about a straight boost to the current cap either, rather than a boost to the previous expansion's cap (boost to 60 to jump into 4.0 content).
Content being easy in general is less about the amount of "new" players at level cap, but more on the developers catering to the lowest denominator. People consider Nidhogg normal to be hard when they're dying to telegraphed aoes that uses graphics from existing fights.. People wipe on Ozma for lacking the basic understanding of not dropping meteors ontop of your party or next to each other, multiple times. The game lacks dungeons that give you a wake up call to actually give a shite about mechanics and hold people accountable to their role (especially the dps) and none of that has anything to do about level-boosting to cap when we don't even have that in the first place.