I'll reiterate exactly why I said this happens in the post you quoted. It's called growing up.
The majority of the market that can play a hardcore game are high school, college students, and recent grads. These people are in this market for roughly 10 years, from the ages of 14-24. These are the people who have more time to devote to video games and also the people who will devote more time to video games.
After these groups of people pass into the next phase of their life, at roughly age 25, they pick up more responsibilities from how the evolution of our human lives works. Whether from financial obligations, multiple or single demanding jobs, having children and wives, joining the military, anything. These people have a much significantly smaller window of time to devote to gaming. They also have a higher average spending capability than the previous group.
But here's where the catch is: The number of people in this group is constantly growing. Even if they weren't, and they were static, they still have a larger age range and thus larger consumer base. These people are typically under this category from 25-65 on average, 40 years, until (early) retirement. Given that people are born faster than they die, the adult group is growing exponentially by default of how long they remain in that category while the smaller group grows at a pretty fixed rate tied to birth rates.
This is why MMOs evolving to what you want to call 'casual' is an evolution. I disagree that it has to be 'casual', as if that is some negative factor. I just think it's going to be more rewarding for the time invested, more variety in a shorter window of time, and easier to come and go. I don't really see anything wrong with that.