That doesn't solve the problem. Anyone acting as a patent troll could just create a proof of concept; it wouldn't add significantly to their expenses in most cases. I don't think you could safely define "marketed product" in any way that excludes such cases, without effectively limiting the use of patents to large companies. Meanwhile, companies building actual products can still create a giant pile of excessively broad patents which apply to your product, and which also apply to any better product that tries to displace yours. That doesn't make them better than the patent trolls in any way that matters to the end user.
By definition, patents exist to prevent competition. If you build a product, it should compete with everything else. If you can't stand competition, get out of the way of those who can.