I think you are talking about really different things. I agree, lots of men would go for younger women if they could, or have a wife and three girlriends, or whatever. Not necessarily a huge age gap, but one where the woman was old enough to be her own person and be interesting to be with.
And for whether they are becoming more common, I don't know, but I would suggest a hypothisis if it is true.There are statistics that suggest that as a group, statistic most women do not date/marry men who are lesser earners or of a lower economic class or less educated then they are themselves. The reverse is not the case.
It's also true however that statistically, young women now academically out-perform men, and IIRC also they out-earn them.
If all of those things were true, in order for many women to find a partner who is on the same or higher level economically, they may find they need to look at an older man who is further ahead in his career.
It also might be that there is more competition for the men who are more educated or earning more, so those specific men have more choice of women and tend to choose ones that are somewhat younger than they are.
If those things were true we might also expect to see more well educated women, and less educated men, remaining single, so it would be interesting to see if that's the case too.