Actually, you didn't explain why characters can instantly know something they never knew before.
That bears no resemblance to anything I said.
Multiclass characters don't "instantly know something" any more than single-class characters who gain a new ability or characters who take a Feat. Don't treat leveling up as magic instant knowledge, but the moment at which your practice starts making a practical difference (quantified as a single moment for ease of use by us players).
Besides which, Fighter, Ranger, and Rogue are all skills any adventurer could develop; they are all practicing their skills and fighting. Sorcerer is natural ability awakening. Cleric, Druid, and Paladin can all represent a growing connection with nature or a higher force. The default flavor for Warlock seems to suggest that gaining the initial abilities
is rather sudden.
About the only one that seems like it might require special training (or practice on one's own, even) is Wizard, and Eldritch Knights and Arcane Tricksters already kind of do that.
Regarding this Prestige Class specifically, focusing on and studying the rune you found, realizing you haven't fully unlocked its power and trying to do so... it makes a lot more sense to me as a natural progression than somebody teaching it. How did
those people figure it out, after all? Somebody had to--why can't the heroes of the story?
I mean, maybe the ability to use new runes without possessing other master runes... ? But even that could just be coming across the rune itself carved into something or printed in a book or such.
I did also say that suddenly picking up a multiclass out of the blue wasn't likely to be a problem
because it wasn't likely to happen anyway. You cut that part out of your response, for some reason.
To that, I said, I prefer a bit more realism as opposed to computer games which tend not to go to that level of realism.
You did say that. I'm still not sure why, since what I said really had nothing to do with that. Which is what I pointed out in the post you quoted here. Seems more like an attempt to insult me.
(Also, the mechanics for learning rune magic are not ever going to be realistic.)