Patents are a monopoly on ideas
That's also a propaganda term. The patent system is flawed but not as much as you imply. You can only patent an idea that's not obvious and novel, which means you need to invest a significant amount of talent, time and money in order to develop this idea. Many people expect some protection of their investment in developing their ideas.
The question is in what would incentivize inventors more, the patent system or the lack of it. I don't see a clear answer to this question.