Wow, what a great discussion. I'll add my endorsement for Economics in a Single Lesson. Beyond that, I want to address the "infinite income for finite investment" question. I spent a while typing up a more robust reply (included further down for bored people), but it really came down to this question:
If you limit how long a given sum of money can earn more money, then What is the alternative use for that money?, after the defined time has expired? And further, what is the BEST use of that money?, and further yet, who gets to determine that? The further you go down that rabbit hole, the more it becomes clear that the original question is one of economic systems, i.e. how the distribution of goods and services is determined.
(In a capitalist system, the answers to those above questions, in reverse order, are "the people who own the money decide what the best use is", "the best use is whatever the owners of that money decide", and "moot, because there is no defined expiration date")
Here is the much longer reply I had typed out before I came up with the above response:
Here's one way to look at it: Let's create a simple hypothetical situation. I buy a house for $100,000 and rent it out for one year. For simplicity's sake, let's assume my profit (after maintenance, etc) is $1000/mo. At the end of one year, I sell the house for the same price I bought it for. I now have $110,000 on hand. Have I done anything immoral? I think most of us would agree that I have not.
Now, I go out and buy a different house, again for $100,000, and rent it at a $1000/mo profit. At the end of year 2, I sell the house for $100k, and now I have $120k in cash. Have I done anything immoral? Again, probably not. Let's say I do the same thing in years 3 and 4. And then for another five years. And then another 10. And so forth. At no point have I done anything unethical or immoral, right? Things don't change when, instead of selling one property and buying another, you simply hold on to one property.
As I follow the reasoning of the original question, it leads to the following idea: "After a certain number of years, I should lose that initial $100,000. It has earned enough for me, and I should no longer receive benefit from having that $100k."
I have three ways of rebutting that argument:
1) I decided to live frugally in order to accumulate that $100k. That means I chose not to buy things earlier in life. A tradeoff. Compare it to, say, going to school and getting a degree. You can't go to someone 20 years out of college and say "you've earned enough money from your degree. Now you can't work in your field any more--you have to go back to unskilled labor." An education is a big investment of your time and money, and it can pay off (in increased salary) for the rest of your working career. Similarly, buying a rental property can pay off (in rental income) for the remainder of the property's life.
2) Let's assume that after 20 years, you're no longer allowed to get returns from that $100k. What do you do with it now? Are you forced to spend it? If so, that means you're forced to buy things that weren't worth $100k to you before. Give it away, to charity, or to the government? They have no claim to it (in my opinion), although you can certainly choose to give it to them if you wish. Burn it? Gee, what a waste of all those hours you worked and scrimped and saved. Stuff it in a matress never to be seen again? Might as well burn it.
3) As others have very eloquently explained, your investment (whether in stocks, bonds, or real estate) provides the capital necessary for economic expansion. Your purchase of stocks/bonds provides capital for a company to expand, or to research a new wonder drug, or develop a robot that will make widgets easier for everyone to afford. (or, in a more likely scenario, your stock/bond purchase frees *other* people's capital to invest in expansion/development/etc).