Some excellent points and insights. You are right, there is only one study out that confirms the egg-producing stem-cells. However, the fact that they were discovered at all is the big surprise.
As for the "grandmother hypothesis," which I have also read about, it seems more likely to me that the benefits of the "Crone Years," post-menopause, were discovered after the fact, and were not in themselves the cause of menopause. And since the women could no longer reproduce to pass on any traits related to this, it could only be a "happy surprise" discovered in old age. I don't think that young humans select for the old-age social/societal value of their future offspring!
Chimps,our closest relatives, can live to be 60 in domesticity, 40-50 in the wild, and they remain fecund/fertile throughout -- still able to conceive and bear young in their late 50s and up to the end of life. Human females usually hit menopause between 45 and 53, occasionally older. A 56-year-old chimp recently gave birth at a zoo in Kansas, and I know a 56-year-old human female who has not reached menopause yet! Humans in domesticity (i.e. modern technological societies) have, potentially, twice the lifespan (120 or so) of a chimp, but like chimps we still typically only live to be 40-50 "in the wild" (undeveloped countries). Our current longevity in technologically developed cultures is very recent. So, it may be a long time (if ever) before our reproductive systems reflect this. Interestingly, even though ovulation seems to stop in humans after mid-50s, women's reproductive systems are quite capable of gestating and giving birth to a baby, as in-utero embryo implantation and surrogacy have proven. It is becominig less uncommon now for women in their 60s to "have babies."
That said, I agree with you that the big question is, "why the heck do human females menstruate?"
It might have something to do with the mobility of early humans, who seem to have actively and constantly migrated around and out of Africa rather than staying endemic to one or a few regions as their primate cousins had. The vagaries of constant migration and transplantation in lands unknown and unpredictable climates (resource availability -- food, water) may have favored a mutation that permitted more frequent ovulation and fertility in females. The hormonal shifts and preparation/evacuation of the uterus may have developed in tandem with and perhaps as a by-product of, that process.
Animals and plants living in regions of predictable conditions (e.g. "expected" annual occurances of drought and plenty) seem to be "programmed" to become fertile/active during periods of plenty, and then go dormant or infertile during the seasonal times of scarcity. Human ancestors and early humans may have lost that wiring due to the inpredictability of their changing locations.
Food for thought, anyway.
I remember reading some information about evidence to indicate that some stem cells survive in human females which lead to some eggs being produced through life. It's a rather newly discovered piece, though, and I saw little more than a mentioning of it.
Regarding menopause -- I've read about the "grandmother hypothesis" which stated that as humans' cultures became more complex, older individuals' stored information became a valuable asset among early social groups. Those females who ceased ovulating incurred the cost of ending their reproductive lives, but also the benefit of a decreased risk of mortality and an increase in how long they were able to continue teaching their children and grandchildren, increasing their survival rate.
Some think that menopause is simply a side-effect of having a reproductive system that worked well enough for the average lifespan of our ancestors but simply breaks down by about age 50. It's possible that the fertile years used to last throughout the average lifespan when that's how long our ancestors lived, but did not extend as far as lifespan increased. While the great apes do not experience menopause, they also reach "human menopausal age" very, very rarely. Perhaps menopause is more the result of maintaining a derived ovarian cycle that simply did not extend along with life expectancy. In any case, we must remember that not every trait or characteristic is necessarily beneficial -- sometimes it's more of a by-product of selection for something else, or simply has a neutral effect. This reminds me of the question "why does menstruation occur only in humans?"
