We can argue the means all we want. This was a set-in-stone design decision by the developers and venting about it won't do anything substantial.

Point of the matter is, Astrologian, Machinist, and Dark Knight all require fairly considerable investments in other classes for cross class skills to begin with. So there is advantage in gating content to them that requires a person to take another class to 50 first, as the player saves time on one of these required sub-classes.

This is no different than so many other games that have long prerequisites to unlocking special classes or features in said game, and griping about it will not change this long-held tradition. Disapprove if you must but this one you won't win on. Yoshida and crew have bit the bullet and accepted the inevitable losses by this decision. It is ups to the players to decide if they are to adapt or be one of those losses.

I will say, however, you're missing out on quite a good game in A Realm Reborn - and that's before even touching Heavensward, which takes it to a whole new level.