I don't know how much the old bike generators weighed, but a modern dynamo hub weighs a pound or more. A cyclist will use around 100W of power to ride 12mph, so a 50% efficient 10W dynamo will cause 20% extra drag.

I'd rather have the solar panels, even though they only charge in daylight, they charge whether I'm moving or not, and I don't have to supply the energy from my legs.