There are two extremes of the monk concept under discussion here. The non-magical Monk would work as a martial class. It's true that martial classes are unduly restricted in what they can do in most versions of D&D (a flaw in the game that goes back to the introduction of the Thief) but, if allowed competence, a Rogue or Fighter build could very easily cover a non-magical monk concept (depending, sadly, on whether the concept is more combat- or skill- oriented).
There we go, actual arguments. Okay. The problem is simply put: that monks are a good 5th class. Their abilities do not fall into the simple 4 man arrangement as many other classes do.
They have some combat prowess of fighters, and certainly some skills or acrobatic mastery as rogues but attempting to build a monk out of each of these is really off. Even creating a monk out of a multiclass would ignore its (self) healing (a cleric ability). And random extra powers that rogues don't get. That psudo-magical feel is elements of wizard without being wizardy or even sorcerery. In short, they are some fighting, some skills. Not really one or the other.
The magical (or mystical) monk is pretty handily covered by psionics, and an 'internal' psionic class more generic than the monk would be a better way of handling that sort of monk concept, along with the potential for other, non-culture-bound concepts.
Psionics has a significant feel in DnD. If you mean 'mind-powered' that is fairly inaccurate too. Examples from 3e would be the psychic warrior or soulknife. A monk is really neither. It isn't a power manifested from their mind as it is a power manifested from their soul. It has a cleric feel (background?) because it is almost religious in nature. A monk should be more enlightened than anything. Again TLDR: psionics already has a feel which is 'psionic' whereas monk is 'mystical' or 'religious'. So not a great fit as far as I see.
The Assassin concept isn't cultural - hired and/or political murderers are darn near universal, sadly. But it is more an activity than a class. A hired killer could kill with weapons, poisons, traps, spells, trained monsters, or whatever...
Hired killers, usually brigands, are common. Killers striking from the shadows are fairly ninja-y. Assassins bridge the two and you think deserve to be their own thing. For some reason though you think monks don't and I don't get that distinction.
Bards are clearly inspired by the Celtic tradition, but they've been scrubbed of just about all of that, and given a lot of arcane magic, as well. They're their own class at this point, I suppose.
Bards have far surpassed what celtic roots. So can (and in certain circles have) monks. Bards are now about knowledge and stories and (most recently) about music. I think that is a change not necessarily for the better but they have come into their own. If they get the right inspiration I'm sure monks can do this too. Beyond that, bards are skill-mages and that doesn't seem to befuddle you as much.
Barbarian implies a culture. Battle-rager might be a better name for the class. The idea is a warrior who taps primal rage instead of developing disciplined combat skills like the Fighter.
Barbarians are those things knocking at your gates but they have acquired the rage ability. Some would compare raging to flurry. But monks are too 'oriental' and barbarians are fine? They aren't too roman for you? Or is something that comes from 'western' society fine and anything from 'asian' bad?
I also notice you didn't touch paladin or druid. I'm sure I could dig up more classes that were in PHB 1. Really anything that wasn't one of the core four. My point is all classes deserve to be there. The asian theme might be the least nuanced example that WotC wants to go with. I wouldn't have a problem if they did the same thing with assassins (likening them to ninjas) but you seemed to object.
Replace magical with mystical and I agree.
What's the distinction, there, exactly?
Magical implies (and often provides) magical spells as a power source. It is the same problem as saying psionic. Both magic and psionics are already a source and while you CAN say monks have power similar to say that all monks come from those sources is, again, inaccurate.
Monks are much more. If magic is best thought of (in the arcane/wizard sense) the understanding and application of forces in the world and psionics the understanding and application of the power of the mind then I would say both are close but wrong. Just as I would say that psionics doesn't accurately portray most wizards.
Now if you would say mystical then you are talking about something else. Something purposely undefined. It is not a power of the gods, or of nature or of anything except the power of self.* Monks gain power from another source, that source is usually inside them, somewhere.
The examples of monks I loved from earlier were jedi. They have a literal power they call the force. Where does that force come from? "Inside all living things," to paraphrase Old Ben. What about the bene gesserit? Again, that is a force from inside themselves, when they learn to become more than they are. When they learn to hone themselves. Monks are the same.
*I would say sorcerers if viewed differently than they are could fall under this same sphere. I'm not saying monks are the only class that should be 'mystical' but I am saying monks ARE mystical instead of merely magical.
