Honestly, there is a way to fix this, because Ishgard is accessible directly from central Coerthas. But, it's a bit of a cheat since Steps of Faith theoretically opens the gateway to Ishgard. Since Ishgardians can obviously come and go without problem, actually entering Ishgard isn't an issue. Entering Ishgard with any hope of the locals talking to you or helping you in any way on the other hand should be a problem unless you complete the Steps of Faith.
So, it could have been possible to allow players to access the new jobs in Ishgard, while at the same time locking players out of any other Ishgard content until Steps of Faith is completed. Basically no doors except those associated with the new Jobs will open for you (assuming you have at least one other DoW/DoM job/class at level 30), and none of the quest givers, or non-trivial NPCs will speak more than two words to you. Because without completing Steps of Faith, you are essentially a nobody to Ishgard. The same goes for vendors, anything other than job specific gear for the 3 new jobs would be unavailable until Steps of Faith is completed.
I'm not sure about the level cap, I would probably have left the level cap at 50 until you complete the Steps of Faith as well. In that way, you can get to 50, obtain the new jobs and complete any raid content available in 2.X with your friends, but you have to compelte Steps of Faith to unlock other areas in Heavensward beyond Ishgard itself, and to unlock the new story elements, story related content and level cap.
It's a kind of halfway accommodation, that still requires players to complete Steps of Faith before experiencing the HW story content, but it also allows new players to quickly obtain the new jobs without having to slog through the entire 2.x story line to get them. By quickly obtain, I mean as soon as they can get one job/class to 30 and survive wandering through Coerthas to the gates of Ishgard.
The down side is that to play HW content and the new raid content, you have to work through the 2.X MSQ, but if you are already leveled to 50, nothing involved in that should be time consuming or difficult. Plus you can practice your new job while doing it.
StormBlood could do something similar, depending on how the new zones and 'city' are structured of course.