Some (not all) character progress would effectively be wiped (made useless or unavailable) by the extensive re-balancing of abilities mentioned in the OP.
A player is currently building a character to be an Elezen Spellarcher (a master of ranged combat, be it martial or mystical) by combining her Rank 40 Archery with her Rank 40 Conjurery. So right now, that character can wield a bow and cast Sleep (at reduced effect) to help maintain crowd control and to allow her to keep her range if the enemy gets too close. Or that character can wield a wand and use enmity reducing abilities like Chameleon & Out Of Sight to keep attention away from herself.
The ability rebalancing and cross-class cut-off of abilities after Rank 30 that the proposed system would require, makes the above character concept no longer possible. That player's effort and progress to build a Spellarcher would effectively be wiped to a certain degree due to the restriction imposed by the proposed system.
Again, as an old-school D&D player, I'm a fan of encouraging creative character builds. It seems like the proposed system takes some creativity out of the hands of the players, and puts it in the hands of that developement person that assigns abilities to the Job Trees. Under the proposed system, the only way to build a character like what I described above would be for SE to make a Spellarcher Job Specialization Tree specifically for it.
Once again, let me know if I am mistaken in this regard.