No, it's not 20 percent of men getting all the sex. This would just not be possible because time and taste are restricted. However, it is indeed only 20% of men who are highly desired sexually by the women they are in relationships with or having sex with. The rest of the 80% of men are in relationships or having sex with women who are either just lukewarm or frigid about them. Or they are not seeing women at all. Women get into these types of relationships for many reasons. Age, poverty, single motherhood, social climbing etc. Importantly, many women understand that they would be a bag of nerves if they paired with a man who was truly top-tier in all respects. Wanting peace of mind is a positive thing.

The women who are lukewarm about their partners will never admit this publicly because it is unkind to admit it, and it also reflects badly on them if they admit to being with a middle-of-the-road man.

Is it 'over' for the 80%? No. Being 'beta' should not be an insult. A 'beta' man is a civilized man by and large. You can definitely be a respectable 'beta' who is high earning and comforting, but at the same time you actually have a spine and don't go grovelling for women's attention.

TL;DR: hypergamy is the fantasy, assortative mating is the reality.