It doesn't matter if the ladder is the perfect representation of skill or not (if something like that exists) when no one plays ladder anymore because they don't want to bother with 100 maps.
There need to be more people playing ladder, and for that, a system need to be in place that people can enjoy. This should be the main focus, not determining the most exact measurement of skill, which is impossible anyway. You can argue real skill and metagame can only develop when people have time to learn the maps (= few maps), or that playing unkknown maps is more of an indicator of skill (= many maps). There is no objective answer to that question, but the question itself is not important anyway. The important question is: How can you get as many people as possible to enjoy the ladder system (without removing the competiative aspect)?