Okay, but why did you buy the book, if you don't care about how the world actually works?
That was my one big problem with 4E, is that all of the fluff was so mutable, there was no way to tell what was really going on there.
One of the things I miss about AD&D was when they described weird corner-case scenarios, like this one. It was probably one of the priest books, or maybe just Deities & Demigods, but it would tell you outright that a god of nature was served by druids (rather than some sort of cleric that could cast nature spells); just as the clergy for a god of magic was made up of wizards (rather than divine spellcasters), and the high priest of the thief god was just a high-level thief.
They didn't feel compelled to invent entire new sets of class abilities to cover obscure situations that would probably never even come up in most worlds.
It is interesting you say this. I think I know where you are coming from...however I just realized something.
In 1e, there was multiclassing and dual classing. It always existed. So did bards! You take levels in fighter, thief and then bard! But more to the point about fluff/ fiction...
A half orc in 1e could be a cleric assassin or a fighter assassin. No one blinked an eye (that I gamed with) or even spoke with. What is more, half orcs are short lived in the old game and not particularly talented in learning, adapting or any mental pursuit. Yet in their short lives they could multiclass...
I don't recall much debate about identity or archetype or lines being blurred.
What I am starting to think and suggest is that some of the sacred cows might be "false memories." And this only just occurred to me today. Now all of this was ostensibly balanced by limited levels (sort of...a half orc in its short life could attain 10th level fighter and 15th assassin!) but the point stands.
The assertion that multiclassing is impure or a challenge to the traditional game's archetypes is off base unless we are talking about something pre AD&D 1e.
Upon reflection, I think multiclassing in 1e is not what needed to be explained--but rather the inability of a human to do so (particularly in light of half orcs, bards and dual classing!).
I have no issue if optional rules in 5e are dropped for taste. I just really disagree that it is fundamental to the identity of the game. The fluff was spuriously immutable from the start. I believe that the restrictions in multiclassing (no humans) and level attainment was a game balance issue first and foremost, however it was rationalized.
I think the current multiclassing and latest edition of the game make this a non issue. I don't think multiclassing is particularly overpowered.