There are lot if things to cover here.
But first up, think about the time-span you are talking about. From little more than coloured water and a tincture of opium, to now is about a hundred years of medical history. Three generations maybe. Coming into a fourth. This is not enough time for any meaningful Darwinian selection to cause any sort of shift in the nature of the human. The vast majority of miracle medical science is only about a generation old. Heart transplants (indeed most transplants) are very modern things.
Next, you need to split the impact of medical advances that aid longevity into those that aid longevity before and after reproductive age. Anything that helps you live longer after you have past reproductive age does not affect the gene pool. Even now, in most modern countries, most people reach their childbearing age with essentially no major medical issues.
However it is worth standing back and asking why this might be so. Clearly infant mortality, and general mortality is vastly less than it once was. The single biggest change in modern technological times to change the life expectancy was not medical. It was sanitation. Sewerage systems and clean water account for the vast bulk of the change. You might have to argue that sanitation is weakening the species.
Probably the next change is medical. Immunisation against many killer bugs. Smallpox is a special and extraordinary example. These two get us to the start of the 1940’s. My parents grew up in an era of no antibiotics. There were the sulphas, and the like. But TB was a killer, and sepsis a major issue too. Both of my parents survived major childhood illnesses that would probably have been cured in a couple of days with antibiotics. Other kids didn’t survive them. But that was two generations ago only.
For the human race to become weaker you need to posit a genetic mechanism that might cause this to become true. Simple random genetic mutation is a very slow thing. It does not throw up huge numbers of weak children each generation.
Weak genes is a very loaded term. What you often have are genes that may lead to a trait that is less suited to the environment the owner finds themselves in. So for that time and place the owner may find themselves under pressure, and less likely to pass those genes on. But those same traits might aid the owner in a different environment. We see some of that in the variation across some races. Peoples who are better suited to the cold, to deserts, and peoples who carry traits that confer some resistance to local disease. Those traits may be exactly the wrong thing in another environment. An Inuit is probably not going to survive as well in the Australian desert as a local. Sickle cell anaemia isn’t a nice condition, but if you live in the malarial equatorial belts, it might be that those genes are what keep you alive.
So, are there real issues? Probably. There is a significant ethical question about people with known genetic disease. Of which there is an ever increasing list. These are conditions that already exist in the populace, and which can become more widespread if those afflicted do reproduce. A good example is Vitamin C. Most animals don’t get scurvy, and don’t need to eat food containing vitamin C. They are able to manufacture it themselves. But we can’t. We have the gene to do so, however it is damaged, and doesn’t work. The likely answer is that at some point an ancestor of the race did receive a random mutation of the gene. Something that would ordinarily have consigned them to an early grave. However they were living in an area and a time in the planet’s history, with abundant fruit, or other sources of Vitamin C, and mutation never caused any harm. And they passed the mutation on. Over many many generations the mutation spread across the species (remember if you go back enough generations we all share pretty much the same pool of ancestors) and eventually everyone got the mutation. Mostly it never causes much of a problem. Unless you are going to spend six months on a sailing ship. But this is an example of where a particular genetic mutation would be well defined as weakening the species.
So, as we progress medically, we are seeing more and more people with debilitating and major issues due to genetic defects survive, and lead some level of normal life. Cystic Fibrosis would be a good example. Appalling condition, usually consigning the sufferer to a death in their teens. But with extended care, new drugs, and better general understanding the prognosis can be better. Or consider haemophilia. Possibly a very interesting example. But many many more.
The risk of better medical care may be that these conditions may manage to pass into later generations, and may, like the Vitamin C mutation, eventually (and this takes hundreds of generations) spread across the species. The answer is probably that it could happen, but is pretty unlikely. It isn’t something that happens in a decade or two (after all the sufferer will have only had time to have one generation of offspring).
Which leads us to the slightly more contentious issue. Often termed genetic counselling. If you know that you yourself have an inheritable condition, or carry a recessive gene for one, what should you do as a potential parent? It is unlikely that a Cystic Fibrosis patient would ever consider consigning a child to the same fate. But haemophiliacs? Carriers of the gene? The answers here are not easy.
