Stats are mainly made up buy rs 2007 gold anyway, so I’ll make some up and pull some vague numbers from memory. In vanilla WoW probably less than 1% of all players saw the end of Naxxramas before BC was released, in fact probably less than 10% of all players ever entered a raid at all.
In BC it got better, with probably over 20% entering a raid thanks to Karazhan, but still less than 2 or 3% saw the end of Sunwell before WotLK was released. In the new raid environment that Blizzard has created in WotLK probably over 50% of players have entered a raid at some point and at least 5% have finished ICC and the next expansion isn’t even out yet.
This means far more players are getting into the raiding portion of the game. When they do they learn all sorts of things that they never would have been exposed to if they stayed playing only solo. This means that in general players get better than they would have been without raiding.