Yes, the problem is that the systems you propose (money that rots f.e.) produce discordination in the economy and if imposed exclusively would lead to poverty.

There is nothing bad in interests. If you have a car and rent it, you are getting a profit because you are renouncing to its use for a while. Same with money. You rent it out and get a profit because you are renouncing to its use for a while. I fail to see the difference and why you consider one ok and the other not.