Neonchameleon
Legend
I'm going to disagree hard and say that I find that one of D&D's active strengths is that it refuses to pick a goddamn straightjacket. And its greatest failures in worldbuilding have been cases where they've done things like hard coding the multiclass rules by species.I sort of agree.
What I think is that, ideally, D&D need to PICK A GODDAMN LANE < honks horn repeatedly >
D&D currently, and this is particularly the case in 5E, does a weird thing where randomly some classes, and some archetypes are in-setting things, which have a definite, comprehensible and reliably meaning, and others are merely mechanical frameworks, which might be used for anything which fits within that broad mechanical conceit - they're almost like 4E's roles, more than actual classes.
If we look at just about any biological rule or sociological system and start dividing it into two groups we'll find it's not as simple as that. That we can mostly do so but there are exceptions. Cases where e.g. the SRY gene has migrated to the X Chromosome being an obvious one.
It doesn't take much thought to see that this would be the case in cases on this thread as well. Even if we go right back to the start and have sorcerers as the nobility of the kingdom? How many groups of rich, powerful, and privileged people do you think are going to consistently and as an entire class keep it in their pants? Seriously? And as for pogroms against unsanctioned casters? D&D societies are fundamentally unstable and any group that tries destroying three quarters of its magical firepower is going to either get taken down from within or without. It's a great plot but not a normal baseline for a functional society.
On the flipside things get invented again and again and stances between unarmed HEMA and Japanese martial arts frequently overlap.
To sum up "In this kingdom all nobles are warlocks" makes some sense and implies oaths and rites for the nobility. But "All warlocks are nobles" had better be a central facet of the power structure of the setting. And "Warlocks are likely to become nobles if they don't die first" makes sense. The one of these that's least plausible is the one where people stick to a lane because neither nature nor people work that way.