Shipwrecked, I have to say you've brought some wisdom to the table with your post. You don't seem depressed to me...simply realistic. What I really liked was your analytical approach about the decline of certain groups over time and tendencies towards monogamy.
As a 42 year-old with a 20 year-old primary, I can tell you, what you're saying rings true. Of course, dating in your age range does help, but that's only one factor. Attractiveness is more than outward appearance, but outward appearance is indeed a major factor at the attraction stage.
I didn't see this as an assassination of polyamory, simply realism.
Me - male, 42, poly, straight, in a serious relationship with Audrey, also casually dating.
Audrey - female, 20, poly, pansexual, also casually dating.