Yes, and I ignored it because it was entirely irrelevant. Starting with a 16 at 1st level and raising it to max at 30th level isn't even remotely the same as having a 5 int at max level.
You are the one who has emphasised that 16 INT - 4 points short of the mechanical maximum - is not enough to be Sherlock Holmes. Yet I have provided an actual play example of a character who started with STR and CON both 4 points short of the mechanical maximum, and now at the end-game of the campaign has a STR still 4 points short and a CON 10 points short, who is nevertheless the toughest dwarf around.
And the reason for that is because actually
playing D&D is not primarily about generating a set of numbers on a PC sheet that accurately model some fictional character. It is about generating
events in play, in the shared fiction, that convey something. If the character with low or average INT nevertheless solves all the puzzles and performs all the deductions, then in that game s/he will play as Holmes-like.
(This conversation is doubly weird because it is the non-2nd ed AD&D players who generally get tarred with the brush of
obsessed by mechanics, whereas in this thread we are the ones pointing out that mechanics are simply an input to play rather than an end in themselves as a measure of the fiction.)
If that's how you run your game, then that's how you run YOUR game. In the books, there is no such limitation on NPCs.
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it is possible that if you add in limitations to NPCs to keep them from being high level AND you start your PC with a 16 (more than 3x the 5 we are discussing) AND you raise that stat religiously, you can be maxed out and the toughest/smartest by max level.
<snip>
Forgotten Realms, Greyhawk, Spelljammer, Planescape, Dark Sun, pretty much all of campaign settings have high level NPCs. It's nonsensical for them not to. The PCs can't be everywhere at once and monsters would have wiped out every PC race a long time before the PCs were born if high level NPCs weren't around to stop them. One evil dragon could have wiped them all out.
Do you start your campaigns with no magic items in the world? After all, no high level NPCs means nobody of sufficient level to create them for the PCs to find. At least not anything powerful.
You are making all sorts of assumptions here about what is the "proper" or "default" way to run D&D.
In my 4e campaign, there are no powerful PCs of the sort you point to - it is a "points of light" game. (There are nemeses statted up as high level, in accordance with the game's conventions, but that doesn't mean they are, in the fiction, functionally equivalent to the PCs. They certainly don't hang around fighting "evil monsters" who would otherwise overrun the world.)
As for magic items - they are gifts of the gods, consequences of PC "charisma" (in the ordinary language sense, not the game stat sense), etc. There are artefacts which were forged in the distant past - The Sword of Kas, the dwarven thrower (Over)Whelm, etc. There are some items forged by NPCs, but given the relatively minor character of those items there is no implication generated that the (purely backstory) NPCs in question are of any great protagonistic potency.
I'm not saying that what I have just described is the only way to run a fantasy RPG, though it is one to which 4e is (in my view) particularly well suited. My Burning Wheel game, for instance, is quite a bit more gritty and doesn't centre the PCs quite as much in the mode of "cosmologically ordained" protagonists.
But even in my BW game,
outcomes in the fiction, rather than numbers on paper, are the things that tell you about a character.