EzekielRaiden
Follower of the Way
Like, when 30 or so of 38 possible sub-classes all use the same rules to cast spells from the same long list in the back of the book, yeah, it's a design risk. It's a long way from 'niche protection,' for sure.
Particularly when official variants are perfectly happy to steal certain aspects of other classes. The Favored Soul Sorcerer comes to mind, and in a certain sense, so do the Bard (particularly College of Lore) and Tome Warlock. Or heck, that one feat which gives you superiority dice. It's not a complete lifting, but we see subsets of mechanics getting handed out, both to particular subclasses of other classes, and through things like feats. Accomplishing, say, 1/3 of the same tasks as another class/es with greater efficiency, another 1/3 from one or more other classes at lower efficiency, and then a final 1/3 that aren't really quite what anyone else does, seems like a perfectly valid rough, overall idea of a decent class. Perhaps it should spin a little different--maybe 20% "same as other(s), but more or more efficient" and shifting the other parts to compensate--but the overall idea of a bit of "like others, but maybe better," a bit of "like others, but maybe not as great," and a bit of "like no one else" is perfectly cromulent design that appears in several places in 5e. One can argue, for example, that the Devotion Paladin has essentially a "horizontal" or "diagonal" slice of the Cleric's abilities, getting some things that are like the general Cleric, and some that are very similar to Light, War, and Life Domain features. Yet it's pretty much inarguable that the Paladin is not as good a healer as the Life Cleric, a somewhat better melee combatant than the War Cleric, and not nearly as spell-y as the Light Cleric--while bringing a couple of really, truly unique features like Lay on Hands and Aura of Protection.
I don't think it's an issue, though. A few class features, some good fluff, a handful of unique-to-the-class tricks, some 'differences in efficiency' and you do, indeed, have a class that feels quite different. And the gain in efficiency (saving page count & design effort, maintaining some consistency) is well worth it.
Agreed. Hence why my preference for judging whether or not something is worth being a class tends to center more on how interesting or clever the mechanical implementation is, and how well-executed the fluff is, rather than on whether the concept itself "deserves" to be a class or not.