In return for the inventor being granted a period of exclusivity, the public receives the long term benefit of the product or process.
Your argument presupposes that without a patent no one would invent anything. I’d be interested in seeing a proof of that.
There are counter examples. For instance, the fashion industry has no patents or copyrights. There are trademarks, but I can copy your design exactly as long as I don’t put your name on it.