I agree to a certain point, but you are underestimating the number of collectors, both male and female, in their late teens to early 30s that have come into the hobby. These collectors buy discounted trades or digital comics for reading, and then spend their $$$ on keys at shows and online. Up until 2009 I was worried about the future of the hobby since it was basically males 35 and up that would buy the big books to fill their runs. It almost seems like the more technology oriented we become as a society, the more widely accepted comic book collecting will become.
As far as big collection sales for comics, if you are talking collections that are mostly copper age and newer, then yes, there are plenty out there. However, OO bronze and silver collections have become much harder to find over the past 5 - 6 years. It used to be easy to land 3 or 4 nice OO collections each year here in Alberta. Now, they are either being passed onto the OO's heirs or have been sold off long ago. Even the assembled collections are not surfacing as readily as they used to.
I think OA is an older market than comics due to one major factor - the cost of OA. You can buy a nice copy of almost any SA or BA key for $10K (except for AF #15, IH #1 and SC #4). That does not really get you into the game for anything of equivalent value in the OA market. You need deep pockets to play the OA game - dropping $2K on a mid grade Hulk #181 gets you the biggest BA book. With OA, that gets you a cover that came out last week.