This figure is entirely different on Classic servers these days.
1% didn't not see Naxx because of it being the most challenging raid designed. 1% saw it due to time restraints, logistics and the release of TBC coupled with such a large part of the playerbase being busy just getting to level 60.
Today, players attack this content with 13 years worth of knowledge on optimization, how the game functions and what the bosses throw at them. 'Nuff said.
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Amen to that.