Did that money come into their hands completely voluntarily?

Likely not. When you buy something you need to survive, can that be said to be a "voluntary" purchase? So when that money ends up in the hands of the owners of the productive machines, they get rich, but are they doing so because of purely "voluntary" participation in the economy?

So then why do I have to pay private individuals for things I need to live, and they don't have to pay back into the system that gave them the infrastructure to earn this money in the first place?

> Did that money come into their hands completely voluntarily?

Yes, if it's in the form of an investment. An investment is a risk, and like it or not, investments are a vital part of any economy. In may cases investments are how small businesses and entrepreneurs get capitalized because banks are much more risk averse. So why punish such a critical segment of the economy? You'll only end up moving the money into other areas that have less benefit for the rest of society.

These arguments would be true if most of our economy and transactions would indeed be centered on things we absolutely need to survive, and the difference between being able to pay and not would be life and death.

It's not. Surviving in any modern first (or even second) world society is laughably easy. So I think we can safely describe most of the transactions as being "voluntary", yes.