I agree. If you look at NPC "fighters", the MM does a great job of using different identifiers. You've got guards, gladiators, veterans, knights, and thugs, and they all act like fighters of various specialties and power levels, but they don't intrude on the space of an actual PC fighter. My heart dropped the first time I saw the druid stat block in the MM, because it flies in the face of that design philosophy and introduces an NPC that implies it uses the same rules as a PC class by virtue of using the same name. I really wish they'd made a firm mandate that monsters and NPCs avoid PC classes in their names.
That didn't bother me so much because while a guard, gladiator, or veteran might not have the features of a fighter (like second wind or action surge) they weren't exactly doing things that a fighter couldn't do narratively. It's not like they had a decapitation strike or something that was blatantly not a PC ability. And for the most part, characters that did mimic a PC class (like warpriest, champion, mage, assassin, bard, etc) either gave a few signature abilities that looked the PC abilities or something remarkably similar. The most egregious examples might have been something like the bard's taunt (which is an ability a bard PC cannot mimic) creating an ability NPC bards have that PC bards can't. But nearly everything else on the bard lined up, and I accepted taunt as replacement for bardic inspiration (which doesn't work well for an NPC) and an ability like cutting words.
It was nice though that if I needed an NPC with the abilities of a bard, I could use the bard NPC stat block as a starting point, add a few spell levels, up HD as needed, and recalc CR and I had a reasonable idea of what I was doing. Now that easy part is gone and replaced with "make it up yourself".
Nonetheless, and correct me if I'm wrong, but nowhere do the rules state that an NPC, irrespective of its name, is mandated to mimic a PC. That's an inference you're reading into things. It's not an unjustified inference, but here's the thing: The players don't get to see the stat blocks you're using. If your 10 hit die NPC wizard doesn't have exactly 5 cantrips the way a 10th level wizard does, they'll never know and it shouldn't break anything in the game. If you've found instances where this actually causes in-game problems, I'd be curious to hear what they are.
As a DM, I used it to know what exactly an NPC was trying to represent. In Ravenloft, Firan Zal'Honan is an archmage. That means looking at his stat block I can tell he's supposed to be an 18th level wizard, just stripped down of fiddly things like Arcane Recovery that is useful to A PC but less so an NPC. In return, he gets a little more tankier (less glass canon like PC wizards are) and some spells are already factored into his stats (like stoneskin). He's simpler to run than a PC wizard, but still looks and feels like one.
By contrast, Candlekeep Mystery has a Master Sage, which has a few "spells" listed as attacks (fireball, shocking grasp, shield) and some in the spellcasting trait (usuable 1/day or 3/day each), but I can't tell at a glance what level of wizard he is supposed to be. They have up to 6th level spells, but due to the lack of slots they can cast more 6th level spells than a PC wizard could. I guess they kinda resemble an 11th level wizard as long as your not tracking spells used.
Worse yet, the Lorehold Professor of Order in Strixhaven is keyword tagged as a "Wizard" but doesn't even have abilities that resemble known spells; he have a weird "contact other planes 1/day" feature, a spectral scroll attack (which does 2d10 force damage and the prof can make two attacks/round with, making its closest spell cousin Eldritch Blast) They also gain a few cantrips and three PHB spells (2nd level, 1st level, and 5th! level) and another "this kinda resembles a spell" attack called weight of history. Its flavorful, it kinda resembles the MTG cards, but its utterly alien compared to the PC wizards this guy is supposed to be teaching magic to. I don't know why he gets the powers he does, or what logic is used to building him beyond "I think this guy should do this!" He doesn't resemble a PC wizard in the slightest, any more so than a mindflayer represents a "wizard".
From what I've seen of the leaks on MotM, it looks like the problem will only be exacerbated. I hope that NPC "caster" classes like abjurer or warlock of the X are built closer to the Candlekeep model (here are some known spells written up as NPC actions) and not like Strixhaven (here are some magical whatever abilities we invented and gave to this "wizard") but from what I've seen on the Bard and Warpriest, I fear the latter is probably more likely.