It isn't just a problem for rich people, it is a problem for everyone who can save money. A growing population means that everything can grow accordingly (businesses, infrastructure, …) so there are good investment opportunities. If you can afford to save money, this means that there are productive things to do with that money, and you'll naturally get higher interest rates (and increasing stock prices, etc.). Higher interest rates aren't all good, but extremely low interest rates (like in Japan for the last ~25 years) aren't good either, especially if you want to save for retirement. Social Security isn't the only useful "pyramid scheme."

It seems to me obvious that if I have a computer, I am more productive than with a stone axe, and it is a more productive asset if I am using it.

There is obviously a surplus from combining labor with capital, and an inevitable struggle over allocating it, but I don't understand at all why people ostensibly think it's obviously all rightfully belonging to one or the other.

If I earn a dollar, and then I collect interest on it, then the interest is really a part of my wages that was withheld. It's not extra. Whether anyone is being cheated of the amount of the interest depends on whether I got the dollar honestly, or cheated or stole to get it, but that has nothing to do with the essential nature of labor or capital.

Rich people are the only ones who will get to reproduce.