I was born in the early 90s, so both wooden pencils and mechanical pencils meant a lot to me. I still have two pencil lead left in my right palm because I accidentally stabbed my hands when I was a kid - pretty sure most of us who have used pencils regularly back then had experienced this.
Eraser was also a big deal to me and my classmates. I owned regular plain erasers, fruit-flavor erasers, and animal-shape erasers before. But I preferred the slightly expensive Japanese brand called "Sakurafoam eraser" [1].
I am not sure when our future generations will stop writing by hand completely, but eventually pen and pencil will see the fate as scribe and Chinese brush pen which are uncommon now.
You can still get them, for example the Lil Juicies:
https://www.ooly.com/lil-juicies-fruit-scented-erasers-set-o...
Quality, specialization and marketing.
Pencils can be quickly swapped for different hardness. Swapping the lead grade in a holder is much more involved. Some artists may even find subtle differences between brands of pencils and may prefer to quickly swap.
Leadholders are almost invariably quite thick. The feel in hand is very different from the slim woodcase pencil.
Whenever I write by hand, I tend to prefer a fountain pen, a mechanical or a woodcase pencil. Writing with a Mitsubishi Hi-Uni or Tombow Mono 100 pencil, for those who can appreciate it, is a cheap luxury. Pencils are higly versatile writing instruments. I'm left handed and heavy handed and prefer H or 2H pencils to prevent smudging. Writers like Vladimir Nabokov praised the Eberhard Faber Blackwing (about 2B on a typical modern pencil scale, although these aren't standardized) for its smooth, almost pen-like glide.
American manufacturers can't compete in lowest price, but they might in best quality. The Germans and Japanese already do.
But surely you could just between multiple holders with different leads in them, which would be equivalent to swapping pencils?
I have a Mitsubishi Hi-Uni 10B woodcase pencil, which is the only 10B rated pencil I could find. As you said, pencil hardness isn't standardized, but it's a very soft pencil, and it does feel smooth compared to other woodcase pencils. But it does not feel as smooth as a 0.5mm mechanical pencil with Pilot Neox 4B lead, which is the smoothest mechanical pencil lead I could find. The Mitsubishi does however write a little darker.
There are competitors which sell twelve-packs of № 2 of similar quality for about the same price, if Amazon is anything to go by.
Pencils are also still basically indispensable in schools.
With this, it is more issue of price, availability, personal preference and practicality. Some folks will seek out such a thing, but most wont.
I found one from Faber-Castell (generally available brand of art pens/pencils). [1] The hardness isn't marked on the outside of the pencil (good for looks, bad for pencil changes) Though it comes with 14 different hardnesses of lead, refills only come in 11 (You don't need such a wide variety most times though). Oh, and they are $11.50 each.
I'd use at least 3-4 different graphite hardnesses in one picture, usually a few more. $45-$60 with hard to find replacement leads for a small benefit that doesn't seem to improve things the way a good paintbrush does. After all, you can just buy Faber-Castell wood pencils (a set for around the price of one) and get the same quality lead.
Now, more to the point, the concerns I would have switching as an artist: some mechanical pencil lead isn't quite the same substance as the pencils, and it is obvious when one erases and blends. Not only that, but I'm pretty sure that even though you can use the side of lead, the shape of the tip is different sort of support than the wood against the pencil. I can see how folks would prefer one sort over an other, so it might not work out.
[1] http://www.fabercastell.com/products/more-products/mechanica...
I can say that lead extracted from a Dixon Ticonderoga produced considerably less smoke than an 0.9mm Pentel-branded mechanical pencil lead when I drove five amps at 30VDC through both of them in turn, as a test of whether a newly arrived bench supply, cheaply bought from a no-name manufacturer, would live up to its rating. Both leads eventually glowed cherry red, as would be expected of any small-diameter carbon resistor dissipating 150W, but I wasn't equipped to measure such temperatures any more accurately than by eyeballing the emission color, which suggests a temperature in the rough neighborhood of 800-1000°F. On the way there, both leads cooked off everything that wasn't carbon, the Ticonderoga lead producing a faint ribbon of smoke that ceased after a few seconds, while the mechanical lead, despite its smaller diameter, released such a robust and prolonged emission that I wondered whether it might burst into flame. Once they'd cooled back to ambient, I handled both leads, and found the Ticonderoga about as sturdy as it had initially been, while the mechanical lead became extremely fragile with the loss of whatever waxes had burned off; I wasn't even able to remove it from the test jig without breaking it several times, and when I carefully loaded the largest piece into a pencil and tried to feed it for a writing test, it was too weak even to stand up to the clutch.
I haven't tried it with leadholder leads, not having enough use for such tools to own any, but I'd expect a result closer to the mechanical lead than the wood-pencil one.