Reskinning does NOT generate new mechanics. That's part and parcel of the definition.
It only uses existing mechanics. Which much exist before begin reskinned.
So, can you restate your point with the starting point that reskinning is using existing mechanics instead of trying to generate and balance new mechanics?
Re-skinning uses existing mechanics, instead of trying to generate (and/or balance) new mechanics. The main problem with re-skinning is that it tries to retro-fit new fluff onto existing mechanics, instead of generating new mechanics that would accurately reflect that new fluff; more often than not, this results in disingenuous representation.
Imagine that you have Object A which is reflected with Mechanics A. It doesn't matter whether you're talking about a class, race, weapon, spell, monster, or whatever. The mechanics always reflect the object.
Now imagine that you have Object B, which is not in the game yet, but you want to introduce it as a homebrew and you're not sure what the mechanics for it should be. Our first check is to see whether Object B is
close enough to an existing object that it should use the same mechanics. If you already have a longsword, and you want to introduce a katana, then you could well decide that it's close enough; you make a note that a katana uses longsword stats, problem solved.
More often than not, there won't be anything close enough, so we move on to step two: Since we've read the books and we're fluent in how the game uses mechanics to represent various objects, we figure out how to translate Object B into Mechanics B by inference. Let's say that we want some way of representing shifter bloodlines, but none of the Eberron stuff has been updated for 5E yet. Well, we know that it's something you're born with, and it doesn't really get more powerful with age and experience (based on our understanding of that reality, as it was translated into mechanics of the previous editions), so that means it should be a race option. And we know how races work in 5E (bonus to one stat, sub-races that give bonuses to one other stat, one moderately-useful passive or limited-use activated ability, and maybe a couple of ribbons), so it's short work. We now have mechanics that honestly and accurately reflect that reality.
To contrast, if you re-skin a barbarian rage to reflect a shifter bloodline, you're not getting the
right answer. Your new Mechanics B are
not an honest reflection of Object B (shifter bloodline), as determined by the best of our ability to honestly interpret how the world works. Your new Mechanics B are just a reflection of Object A (barbarian rage), which has nothing at all to do with Object B. It's balanced, sure, because it doesn't change any of the mechanics from if you were
actually modeling a raging barbarian; but it doesn't
mean anything, because you had to artificially contrive the correlation between the reality and its reflection.
Hold on a second. Please look to your Monster Manual. There are dozens of example in there of various things that are not the same as if they were built as a PC using the PC rules. That NPCs and monsters do not follow the same rules as PCs for creation is well established. 3.5 was the last edition that tried that.
The PHB only describes player characters. It explicitly does not try to encompass everything in the world. An argument that assumes the MM is not part of D&D and that all NPCs and foes in 5e must match PC classes in order for the world to be understood does not fly.
NPC classes are just PC classes that have been trimmed onto a note card for ease-of-play. If you wanted to
accurately represent that level 9 Mage NPC, you would write them out as a full level 9 Wizard; but if you don't care about
minor loss of accuracy, in order to
vastly speed up the creation process, then it's close enough. This is explained in the DMG.
The PHB describes the part of the world that is relevant to players, like how dwarves and druids work; it is the definitive source for how those things work within the world, unless the DM goes out of their way to change them. Things in the Monster Manual are
also a part of the world, if the DM decides to include them. If you have orcs in your world, then the Monster Manual tells us how orcs work, unless the DM goes out of their way to change them. If this world has orcs that are druids, then refer back to the PHB, which is the book that tells us how druids work. If the world has orcs that are shamans, and a shaman isn't close enough to either a druid or a cleric for them to just use the same rules, then the DM has to determine which mechanics to use to best represent that reality (based on their understanding of what a shaman
is, their fluency in how 5E uses mechanics to reflect class abilities, and possibly the streamlining process by which a full class is summarized down to a note card for ease-of-play).
