being asexual is nothing to do with an individual choice to be celibate.
And people are not "starting to see themselves as not being sexual" it has been 1-2% of the population since the beginning of history.
And in "evolutionary terms" as you put it, asexuality ( and homosexuality) are likely to be an evolutionary advantage to society/tribe/family as it means there are always likely to be some adults not reproducing, and the amount of nurturing and care needed to raise a human child or sibling group means that no child can rely on just their parents.
Aunties and uncles without children are available as spare careers and back up
This is believed to be the reason behind homosexuality rising as you go down the birth order... later born children are more likley to be gay, less likely to have children, are more likely to be available to help raise, guard, protect, feed and teach their older siblings children. If you are first born, you are less likely to be gay, because it may be that you never have siblings that want your help in child care
There has always been in human society a pool of LGB and asexual adults, who for the 99% of the life of our species, have been the extra adults needed to raise the next generations
Also, the evolutionary reason for the menopause
so nothing worrying here at all, in fact, this has helped our species survive.