I don't know much about light pens, but it seems from the Wikipedia article https://en.wikipedia.org/wiki/Light_pen that they are primarily intended for use with CRT monitors, which limits their utility nowadays.
Touch screens are obviously very much modern devices, and, though they're far from ideal as the only input device, I can buy that, when used in conjunction with a keyboard, they are a good replacement for a mouse or trackpad; but don't they suck up power in a way that mice and trackpads don't?
Yea light pens were originally a way to add something like a touch screen to CRTs way back in the "old days" long before alternative displays were feasible let alone better. As far as touch screens using power, they don't as I've known, or at least they don't have too. the same tech that makes up touchpads is usable to make touch screens (and in fact it is on your smartphone and tablet). That may not have been true back with the resistive touch sensors such as those on the palm pilot and such.
Touch screens are obviously very much modern devices, and, though they're far from ideal as the only input device, I can buy that, when used in conjunction with a keyboard, they are a good replacement for a mouse or trackpad; but don't they suck up power in a way that mice and trackpads don't?