Why does the proportion matter? Yes, more casual gamers entered the scene - this is to be expected as computers before more accessible, but the number of people willing to play complex strategy games has overall gone up.
I think this is the crux of the artists' complaints: you can't conquer a mature, diverse market with a one-trick pony. Back in the day, the 300k audience your game (or song, or art) had was a very large chunk of everyone that's into a new medium. Today, a sequel to that work could gather a 3M audience, but then the artists look at the market being 300M strong and weep, instead of being satisfied with "merely" 10x the audience.