Yes modern medicine changes the rules a little. However it does not stop young males (in particular) from engaging in various hi risk activitites like base jumping street racing etc. which do serve to remove some individuals from the gene pool.
It is also a very western perspective. Sure in OECD countries we have gernally low child mortality rates but this isn't true everywhere in the world. Evolution is still going on in less advanaged parts of the world.
Evolution is till going on in 3rd world countries, particularly the industrialized ones where pollution goes unchecked and people who can't handle contaminated air and water have a good chance of dieing in childhood.
If I recall correctly Communities which have engaged perpetuated the model of older fathers having many children (ie the Mormans) have an increased incidence of some congenital conditions due to their genetic makeup being dominated by a dozen or so of the religions founding fathers. Heck even they have moved away from polygamy for the most part.
Humans have a complex social life and the definition of fitness in a complex society is different to what it is among other mammals. having greater physical strength is not necessarily a deciding factor.
And we still have a lot of social groups which self select to remain isolated from the broader human population.
PS short sightedness has been shown to be largly caused by environmental factors and not genetics. If you live in a city, and very rarly get to look at anything further away then accross the street, you have much higher changes of needing glasses than your twin, who is brought up in the country. and gets to look out to the horizon every day.
K
