Go into any used bike store and you'll find nothing but mixtes. They are slow sellers, and plentiful. There definitely is still something of a chauvinistic attitude towards them. And women cyclists on average are still less plentiful than men, and they too sometimes choose a "men's" bike too...
To echo fellow bay area resident, jade408, that is not my experience. Not only have I found mixte bikes easier to sell than comparable men's frames, but I'd rather deal with a female buyer. Less BS and flakiness, but that's not really the point here.
From a fivethirtyeight.com post asking
"How many Americans don't know how to ride a bike?":
"I expected a gap between male and female non-riders, and there was one: 60 percent of women and 42 percent of men said they never ride a bike even though they know how — I’ve looked at the various reasons for that in the past. So based on that previous research, I expected a similarly big gender gap in the percentage of Americans who don’t know how to ride a bike. But as it turns out, there’s barely any difference between the share of women who say they can’t ride a bike (6 percent) and men who say the same (5 percent)."
So, yes, perhaps there are more male riders. However, the disparity may not be as great as you think. More importantly, the ratio of mixte frames vs. traditional diamond frames produced is far below the ratio of female to male riders. As a result, even conceding the premise that most males won't buy a mixte, there is a greater ratio of (largely female) buyers looking for mixtes vs the ratio of mixte bikes on the market as a whole.
All this is to say that the laws of supply & demand suggest that mixtes in general, and especially high-end mixtes, are indeed collectible. They may not always have the cache of a bike with racing pedigree, but there are plenty of mid- to high-end mixte frames/bikes, especially in the French constructeur market, that command top dollar.