It could also mean just spreading out. So less dense civilizations, but the same amount of people.

In could, but that's generally not what it means when applied in this way. You don't thin out a a herd (which is where I think that phrase is most often used in English) by spreading it over a larger area, but by removing a percentage of the individuals that make up the herd.

If you have lived a good life, is dying a problem?

Starving to death together with the rest of your tribe - your parents, your siblings, your friends, your kids - must be mentally excruciating. Famine has been considered one of the worst curses of humanity since the dawn of written records at least.

How happy are we who have never experienced that! How would our ancestors envy us!

Truly if one was to know the excruciating pain of those who have gone before these things of today thought of as worthy of attention would fade to nothingness.

Would our ancestors envy us or be happy for the fruit of their effort? There are setbacks and frustrations about the slow state of progress sometimes for certain. Maybe they would also be sad for the material orientation of many people and the loss of spirituality that gave them much hope and fortitude.

Dying from starvation (and watching the people you love also suffer and some die from it)? Yes, I'd say it's a problem.

Or, If you like something, would you want less of it?

I for one think, this is a subjective question.