I've thought about that watching old movies from even as late as the '30s - that every single person I'm watching (scenes with 10s if not hundreds of people) are long dead.
Every time I watch old films or start pondering mortality I then think of life longevity augmentation which is probably going to happen in the next 40-50 years (if not sooner) and then wonder if a. I'll live that long, and b. what a shame it is all those people are dead.
Then I start thinking of all the differences we'll have when longevity comes about, such as being able to travel intergalactic distances, declining birth rates, and placing a higher price on the human life. It's an interesting thought exercise.
What about Youtube? Never in history could people so easily share their lives with the whole planet and it being archived very well (and searchable). Think about the impact it (already) has on the future, when people somewhere hundred years from now can look back at so much of human history in full color and sound etc. We as humans are currently recording so much for future generations. Half of it may be crap, but on the other side there's also a lot of good stuff.
Then I start thinking of all the differences we'll have when longevity comes about, such as being able to travel intergalactic distances, declining birth rates, and placing a higher price on the human life. It's an interesting thought exercise.