The approach I have tried to represent is flexibility.
2e just tried to address this by hundreds of prestige classes, but the class restrictions remained.
In the example of the thief retraining as a mage, the point I was trying to show is why they could not switch between the two and when appropriate, say, train to a competent thief level, do a bit of training as a mage to learn the Knock spell, then switch back to thievery.
All this could be done without the restrictions of the dual or multi-class rules, which I think were added on to the existing class rules that were inherited from original D&D, I have tried to maintain as much of the existing rules as possible, and accommodated the "specialist" classes as well, as it happens I wasn't eschewing the training aspects of DMG 86 either, this is purely creating an alternative to the class restrictions.