I am Gen X not Gen Y but I see no reason why Gen Y will not follow a similar path as they age.
I’m estimating I had about 20 friends living in SF while in their 20’s. Great times. They all got married, started having kids, and every single of one of them moved out of the urban core. Typical reasons of course – more space, schools, safety, more kid-oriented environment. Access to bars and restaurants move pretty far down the list of priorities.
Same exact thing with a handful of friends in NYC and Chicago and then a huge number of friends in DC all moving out to suburbia. Same reasons as above.
This is a huge country with people choosing to live in many varied settings and there is no right or wrong as far as I’m concerned. I know plenty of very smart, fun, cool, hip people living in stucco boxes in suburbia because the time came to make the trade based on changing priorities. And they are living well and enjoying life.