Of people living in the Western world at least, and in most cases 3rd world countries too.

You'd have to be very silly to say that the life of someone in the lowest rung of the working ladder nowadays(e.g. a McDonalds worker) is not better than the life of a peasant in feudalistic times.

Peasant feudalistic times are so far in the history that they are irrelevant.

When people talk about quality of life going up and down, they mean compared to last 20 years or something like that.

Comparing with a previous generation or two, how does a 25 year old's life compare now compared with 2001, 1981, 1961 and 1941

I don’t think you can make that generalization

When discussing short term changes, it's logical to compare to short term pasts.

I get your point, but at least in this case we should probably compare to quality of life just before these chemicals were in use.