If the numerical keypad was that useless, I'm pretty sure, keyboard manufacturers would stop including them.
Just like adopting QWERTY right?
I think with keyboards, not having it as a general keyboard would throw people off. A lot of people here have adopted the idea that it's ok not to have it, and while data entry might be really nice and cool, even entry level front end developer jobs (and I'm talking very entry level...no unobtrusiveness, no accessibility, no validation, etc) won't need the number pad. I'm on a computer for more than 12 hours a day, and have been for the passed 8 years, and the only thing I've "missed" since I switched to my HHKB is by hitting win+r, quickly typing in 'calc' and doing some quick math stuff with the number pad (and even then I still get it done, it's just a little slower). If my main job was data entry, then sure, I'd probably score a number pad. But it's not, in fact short of accountants, or dead end data plugging or converting big stacks of paper to a digital format (no offense if that's anyone's job), it's not really quite all that valuable.
I honestly think normal people's (not us) first reaction would be "wait...so I'm paying how much for half a keyboard?" and maybe for a lot of companies like Logitech and Microsoft, retooling for tenkeyless models wouldn't really be all that valuable in the end. Much better to just design the keyboard with it than have people gripe about it missing.
On top of all this, it's very tough to convince people that, if you're not using it, why have it? The way I type and navigate with my mouse has completely changed since I went to a Happy Hacking. I no longer have to type with my wrists off center. And I no longer have to adjust the pivot of my right elbow to reach my mouse. Everything just feels much more natural. It's not one of those "hmm, yes I can see that I should give it a try", it's more about just getting a tenkeyless model, and if you think you can really survive without it, then try to look optimistically at the benefits.
And if you're an accounting, or excel data entry PROFESSIONAL, or whatever else that would use the number pad, wouldn't you want an external one anyways? Being as it's probably a hell of a lot more comfortable having that positioned somewhere else than to the right of arrows?
I think people are just afraid of changing a layout too much because of it's familiarity, and losing it would make them work slower or less inefficient. Although, I can probably tell for the rare time that I would need the number pad, I save much more time by traveling less from my keyboard to mouse now that it's so close.