As a dev from the outside looking in, it's probably not worth the effort. Like CNJ/WHM and others are tied together for a variety of reasons in the game. They also built this trust system on-top to simulate players like conjurer and gladiator trusts, and you have trust story NPCs that are classes, or a sub-class of one. It just doesn't seem like a good use of time after all these years to say no job = no dungeon, and then have to write exceptions for all kinds of scenarios you may have to visit years later.

From jobs-only at 3.0+, broke the cross-class system in 4.0, then made role actions instead. Basically they did things to put dirt on-top to make it not as common or viable in the past, like when no echo, classes only savage A12S was cleared for fun.