I don't think people are discounting or ignoring the experience of those who are currently adults who had elderly parents and found the experience of caring for them and other consequences difficult. However, as we shouldn't discount those very real experiences, we also shouldn't discount those who talk about having older parents and it not being an issue, or the fact that people mention other qualities that parents might have which have a far worse impact on childhood and the lives of children into adulthood. It would also be wrong to see people becoming mothers at 40 today as exactly the same as them becoming mothers at 40 in the 1960s or 70s, or to imagine the children born to older parents today will have exactly the same experiences as those who are now well into middle age. Although risks of being older are still higher than for those who are younger, most people are in better health for longer and live for longer, plus of course having older parents will increasingly be common rather than rare and unusual. With all these things, downsides and risks have to be identified and weighed don't they and then a decision made - and not everyone will reach the same conclusions because there isn't a categorical right answer. Because some people found being the children of older parents difficult doesn't mean all will, nor does the fact some found it no issue mean all will.
If I was 40 ish and hadn't had a child and was really keen to be a parent, I think I'd think to myself, about all the pluses and all the potential risks, their severity and liklihood - I guess if carry out a risk assessment if you like.
Risks
yes there are risks of me dying before my children grow up - very awful if it happens and very low risk. yes, there are risks of me entering poor health and dying whilst my children are in their 20s or 30s - fairly awful it it happens and reasonable risk.
Other factors
Am I in a stable relationship, am I financially stable, am I capable of bringing up a child, am I capable of engaging with a child and loving them? do I really want to be a parent? Can I see a child having a happy life and enjoying a relationship with me?
And I'd balance the risks, which are the severity and the liklihood against the positives, and reach my decision. Everyone will weight the risks and positives slightly differently and therefore reach different conclusions.
Some may decide that the risks are too great and the chance of earlier illness and death is just too negative even bearing in mind all the positives for the parents and also the child that would then exist. Others might decide that although there are risks, they are low enough and uthe positives so great, that the positives outweigh the risks and make it worth going for.
Personally, if I were 40ish and childless and wanted a child badly, I think I might weigh these risks and decide to go ahead, but I'm not saying all should or its the right answer because there isn't a right answer. I'd recognise the risk but see it as low. I'd also try to mitigate the risk as far as I could - so I'd keep working on staying fit and healthy and I'd put measures in place to help with my care if I were to become ill in my child's 20s or 30s and I'd work hard on making sure they had a good support network around them so any burden could be shared/they had support. And I'd take the risk because I think that the numbers who experience an older parent dying in their childhood are low, those experiencing illness and death of parent in 20s and 30s are higher but not totally devastating for most, and mostly becaue I believe the benefits and enjoyment BOTH parents and child would get from the parental-child relationship, however long it lasts will be good enough to make it worthwhile. That's not to say there won't be costs/disadvantages - there are in all relationships of all types and with all ages, but is there a net benefit - and I don't just mean to parents, but to children too. And I personally would go ahead becaue in my situation and with my assessment of the risks and positives, I believe overwhelmingly that I could be a great parent and my child could have a happy life.
Of course, I could be wrong, but I can only act based on the information I have and on a reasoned assessment of it all. We all have to make decisions based on partial information and uncertainty all the time - and sometimes the best case scenario happens and sometimes the worst does. If the worst case scenario happens, it doesn't make the decision wrong. And I think we have to assess the risks and then live our lives both in the reality of the those risks, but with hope and looking forward and to enjoy our lives, not in fear which stops us from doing anything or living negatively.