In the past, this was a more common thing. Disease, injury, and war killed a lot more people than they do today. I wonder if people did cherish relationships more then than they do now.

I think you're on to something here. I wonder if a gradual loss of empathy is a product of the evolution of a first world country or advanced economy?

People die less, and without the threat of random deaths, life itself ends up valued less overall.

Humans seem less inherently empathetic toward emotions they haven't ever experienced close to themselves, loss being one of millions of negative human emotions that when experienced less, is less able to be empathized with at a macro level.

Perhaps this partly explains the current political climate and recent spike in authoritarianism among the average citizen in the US. Maybe we've hit a point where the life necessities and healthcare needs historically only enjoyed by the powerful are available to almost everyone and as a result, life expectancy has goes up (meaning statistically less people are dying in any given time frame). People start feeling subconsciously both slighted (economically) and invincible (biologically), and assume everyone else should be invincible too and there is no room for softness (empathy).

Perhaps this is even how humanity self-regulates. What if "dark ages" where reversals in progress are experienced are caused by this ebb and flow of the spirit of cooperation and empathy?

There is a nice, short (12 pages!) scholarly article on this topic called "Did the Ancients Care When Their Children Died?". Evidently, this question (and the more specific question of child mortality) has seen quite a bit of study. I strongly recommend that article, but if you can't access it let me give a sketchy recap below.

The article makes a few points. First, child mortality in Greco-Roman times was very high, maybe 30%. Second, art from that time has many references to the terrible emotional pain of losing a child -- grief seems to have been common. Third, handing over child-raising duties to another party (wet nurses, or even entire foster families) was also far more prevalent than it is today. Fourth, the death of a child was often mourned in a communal, ritualistic way, but it was also not uncommon to simply bury a child in the home with little fanfare.

These things are all hard to square, but the rough conclusion I got was: even when mortality was high, losing a child was keenly painful if that child was loved in the first place, which was less of a given at that time. However, even in that case, mourning and grieving -- by being both diffuse (by community) and following predefined rituals -- seem to have been more self-contained. In other words, the pain came, and then it (mostly) went. Child mortality being such a common experience, there were established societal ways to deal with it (plus your friends and family knew what it was like), and this made it genuinely easier. Your post makes some similar points.

My own take is that today is the worst time in human history to lose a child. Communities are more fractured, there are far fewer people who understand the experience, and parents have every reason to expect to never see any of their children die (and are consequently less prepared for it). Plus, in ancient times there were at most a few ways you or anybody around you would know how to raise a child, so a death that came from that process was in some sense excusable, or somehow just a stroke of fact. Contrast that with today, where we have maybe more freedom and choice than ever, and for every decision made we can find an argument against it -- there's just less certainty of having "done everything right". It's an ugly price for agency.