Everybody gets presbyopia in their 40s or so, that's 100%. If you're nearsighted (about 30% is the number I've seen) you have to take your glasses off, if you're not (everybody else) you need reading glasses. Either way, it's more difficult to focus on tiny things.According to AdMob, almost 20% of iPod Touch users, and over 45% of iPhone users, are over 35 years old.
I can't find the stats on the percentage of people over 35 who need reading glasses, but even if it's only 10%, that would amount to over a couple million current iPod Touch and iPhone owners who would benefit from a device with a larger display, not even including Apple/Mac customers who didn't yet buy any of the current devices because they found the display too small to easily read much.
That's right, everybody, anybody, you Mr. or Ms. Reader, if you're lucky enough to live until middle age, you will need either bifocals or reading glasses.