EzekielRaiden
Follower of the Way
Whether or not you are sold on it is not, IMO, the most relevant factor. Whether or not it is a feature that people expect of a game called "D&D" is a more relevant factor.
Beyond that, though? I doubt I have much chance of swaying your position, but I think of it this way: Somewhere around half (possibly a bit more) of college graduates end up getting a job that doesn't actually relate to their major, or working independently in a field rather removed from it. Possibly one of the most obvious examples, Rob Heinsoo, has an anthropology degree (focusing on the anthropology of faith/theology), despite having been the lead designer of 4e and co-lead of 13th Age. For an example pretty relevant to me and my interests, one of the hosts of the YouTube channel "Overly Sarcastic Productions", which is among my favorite channels, has a mathematics degree, and has quite literally said on at least one of her videos, "And no, this is not how I expected to use my mathematics degree, but here we are."
That's what multiclassing represents. You learn things from one field...and then find that your skills develop in a different field. That doesn't mean you forget what you learned. Heck, you may even re-apply those things. I, personally, have a math/physics background but a strong bent toward philosophy, things that (in the modern era at least) are often pretty widely separate.
So "multiclassing" represents that real-world situation of interdisciplinary study or starting in one field/area/knowledge and then developing in another. Sometimes that goes somewhere. Sometimes it doesn't. Sometimes, the whole is more than the sum of its parts. Sometimes...it's at best the sum of its parts. Oftentimes, you can find surprising ways for how what you knew from elsewhere applies to what you're doing now.
I don't see why this couldn't be a subclass. And this presents precisely the use case for why subclasses/archetypes are useful. You have presented this as "well of course the Assassin can't actually have the James Bond panache as well. That would be ridiculously over-the-top, to have ALL the cool things an Assassin should be able to do, and ALL the cool things a Superspy should be able to do."
But this is the very place that subclasses/archetypes give us the ability to do that. The thing you think of as a "proper" Assassin is a character who has both the basal Assassin class, and the features which make the extra especially lethal and dangerous. Your "James Bond" concept has only the basal part--making them decently competent at basal Assassin things, but not a specialist. Instead, they acquire benefits which are of a social bent, stealth not as skulking-in-shadows, but as disarming rhetoric, debonair sex appeal, and resourcefulness in restricted situations where your tools need to be unobtrusive, even invisible.
To give you an example of what I mean, let's consider the core mechanic of the 4e Assassin (specifically, the "original" Assassin from Dragon magazine, rather than the "Essentials" version, aka the "Executioner" Assassin.) The core mechanic of this class is the "Assassin's Shroud", which basically lets them build up to a massive damage hit when they choose to expend their shrouds (a target can be subject to a maximum of four). By default, the Assassin can only apply one shroud to any target in a single round as a free action, but there were other powers, usually encounter or daily powers that took actions, which could increase the number of shrouds applied. I could easily see a subclass of this Assassin that can exploit their shrouds for skillful, manipulative ends: a bonus to Deception, Thievery, and Diplomacy, for example, equal to the number of applied shrouds--but applying a shroud while not in combat has risks, since even though the shadowy magic is invisible, its effects can be felt and possibly resisted. But nothing ventured, nothing gained; the "Superspy" Assassin has to have given up something else in exchange. Perhaps they can't apply shrouds in combat as a free action, so they're stuck at a much slower rate of damage-dealt growth. They can still do all the basic stuff, but they either need to set up a target well in advance (much like a sneaky spy would do!), or they need to bring to bear some other resource (perhaps some of their spy gadgets?) in order to make fullest in-combat use of their Assassin abilities.
Point being, it is quite possible to have a meaningful and worthy sacrifice of ability in order to fulfill a clearly related thematic space, without destroying useful niche protection. Absolute, ironclad, never-to-be-violated niche protection has its own foibles, after all--remember the problems of having to persuade someone in the group to make the sacrifice and play "Brother Bactine"!
What does this do to wildshape? You won't be able to sell folks on a Druid that cannot wildshape--that's just non-negotiable at this point.
Plenty of names in D&D are terrible. "Paladin" refers to a knight of the Palatine Hill, which is something that shouldn't exist in any world but Earth. (Indeed, "palace" also shouldn't be a word in other worlds, but it is.) "Cleric" is a terrible word tied to specific (and mostly Abrahamic) traditions, and actually has a bunch of non-faith-related meanings that are even less like "armored priest". "Barbarian" takes the cake though, far and away worse than any other D&D name, because it's literally a derogatory slur levied against foreign language speakers.
If your argument is that the names used in D&D are excessively tied to narrow cultural contexts, then I'm afraid that ship sailed literally like 40, 45 years ago. It's much too late to turn things around now. D&D has caused enough influence on the fantasy genre that "Druid" now means what D&D declared it to mean, more than it does the original cultural context it came from. It's no longer a self-fulfilling prophecy: it's a self-sustaining narrative. It would be like telling people to sing the alphabet to the tune of "London Bridge Is Falling Down" (which I can do, in two slightly different ways!) rather than the tune of "Ba Ba Black Sheep", or to use circular-shaped graduation caps rather than square. It would just feel wrong to most people, and they'll reject it without really knowing why.
