One of the most insidious and dangerous assumptions we have is the silly idea that human history has a direction. That in some meaningful way, life in the 21st century is fundamentally different (even "better") than life in, say, the 14th century, or the 21st century BCE for that matter.
The second sentence in the above claims that not only has there been an improvement in the last 600 years, but that nothing much has changed. There are
literally hundreds of counterexamples, mostly technological, some social.
You move on to say that longer lifespans aren't intrinsically a good thing.
As for your specific example, clean water and sanitation are better if we take it for granted that longer lifespans and higher population density are better.
Sanitation is an improvement if you have
one human involved. You don't need a high population density. And it's not just the length of the lifespan, it's also the part where you don't shit yourself to death from dysentery.
Here's another example: improvements in agriculture and transportation mean we can amortize the effects of local crop failures. That means the tribe that worships Enfen-Loqa of the sevenfold tongue, doesn't have as much of an incentive to kill the proselytes of Amur-Hoth, just to ensure their access to the food supply.
3. The final question about what we have gained, as might be evident if you remember that it is the last line in a larger piece and not just a singular lonesome question posed all by itself without context, is asking whether our technological progress has made a difference in the fundamental, innate feeling of being a human being or our chances of being fulfilled as a member of society.
Your chances of being fulfilled as a member of a society (whatever the hell that means) are somewhat lessened if you're dead.