Have solar panels become that popular in the last ten years, and are people retrofitting old boats? All my prior great lakes sailing experience was on boats that would use the diesel motor to recharge batteries.

Yes and yes. Being able to have all but guaranteed power to float charge batteries, run bilge pumps, etc, etc, really makes disuse a much easier problem.

And batteries like the cyclic nature of the sun much more than a constant on float charger.