Clearly the majority of guys who fantasize about so-called MILFs are pursuing this strategy out of desperation: they're hoping that she's been neglected for long enough that their sorry ass looks like Prince Charming.
And on a Darwinian level, having kids by her would be a longshot if she were nearing menopause, which makes the strategy look even stupider.
But here's one reason why it might pay off under the right circumstances (for one thing, you'd have to be a desirable male, not one who'd waste a lot in the pursuit). When girls are between roughly 15 and 24, it's very hard for them to look repulsive. Sure, there were ugly girls in your math class, but were they as bad as the decrepit 40-something who works at the supermarket cash register? Their eyes are large and clear, the skin everywhere is bouncy tight, they've got subcutaneous fat where it belongs, nothing is sagging, and their hair is in great condition. There's much greater variance in appearance among 40-somethings, mostly due to the low-ranking ones really hitting rock-bottom while the high-ranking ones don't fall so precipitously.
Now, the opportunity cost of pursuing and mating with a stunning 40-something female is pursuing and mating with a stunning 20-something female. In fact, it seems like that opportunity cost is worth more than the long-shot pay-off you'd get with the 40-something.
But let's say you're interpreting physical appearance as a signal of good genes. Then you may want to go with the older one. After all, a 19 year-old hasn't been through that much environmental stress, so if she looks good, she could have gotten lucky so far, or the environmental insults just haven't accumulated to a high level just yet. Her good looks are a fairly noisy signal of good genes. However, if a woman looks like Audrey Hepburn in Wait Until Dark or like Monica Bellucci does now, that is a pretty noise-free signal of good genes. They've been subjected to several more decades of environmental stress, and they still look pretty good? You can't get lucky for that long, and you can't fake it either, not even with plastic surgery.
So you're trading off one type of risk against another -- the older stunner has a lower risk of having bad genes, given her good looks, but a higher risk of not conceiving. The younger stunner has a lower risk of not conceiving, but there's a higher risk of her good looks misleading you about her genetic quality.
Given how males of all ages react to the two stunners, it was probably worth erring on the side of youth. But going after Penelope Cruz at 35 could not have always been a bad bet since the preference has survived for so long, albeit at a lower frequency than for chasing after a stunning sorority sister.