I view a different stat block format as a purely meta game construct that doesn't impact the story, and consequently I have no trouble with, say, one kind of archmage that is an 18th level wizard using the phb advancement rules (a PC) and one that uses the CR 12 MM statblock (an NPC).
Agreed.
I have run D&D modules using a range of different systems: I've run B/X modules using 3E and 4e, AD&D modules using Rolemaster, 3E/d20 modules using RM and 4e, etc. Th mechanical conversion that this requires is something that I've always treated as a metagame execise, not as a rewriting of the imaginary world of the story.
Changing statblocks between PC and NPC is just another feature of that. For instance, for a PC it is interesting for the player to have flexibiity in making choices about bonus actions, multiple attacks, activating self-buffs, etc. For an NPC it can be quicker and easier just to bundle this stuff into basic expressions of combat effectiveness, like AC, attack bonus, hit points etc. Ultimately, what is the difference, in the fiction, between a bonus to hit from DEX (lets you strike with speed and precision) and a proficiency bonus to hit (lets you strike with speed and precision)?
However, what I don't like is inconsistency or change just for change.
When you frame it this way, you will get some people who don't agree with you, because they think that the rule isn't "change just for change" - in part because they think it is not a change (and will point to berserkers, sages, mercenary officers etc in AD&D as earlier example of NPC generation following different procedures), and in part because they think that it not "just for the sake of change" but rather well-motivated.
Also, what you call "inconsistency" some people are going to see as a type of excessive focus on rules over fiction. Within the fiction, after all, what is the difference between someone with 1d8 +4 for CON, for a total of 9 hp, and someone else with 2d8 and no CON bonus? None that I can see. So whether your focus on HD or CON as a way of generating your NPC's hit points is mostly a matter of metagame concerns - what else, in the game system, is associated with HD or with CON - rather than a matter of fidelity to the fiction.