The GP is saying that a regulation’s unintended or higher-order effects can create a net negative impact on society. Why “should” such regulations exist?
I'm not sure it's net negative in the case of the GDPR. As a sibling (cousin?) poster said, good riddance. No company should be allowed to mismanage my personal data, big or small. Until I see evidence that GDPR compliance is cost prohibitive enough to suppress startup formation, I won't be convinced that the tradeoff isn't worth it.
The other issue is philosophical. You seem to want proof that a company should exist and require extraordinary evidence against a law. I want proof that a law should exist and require extraordinary evidence against a company. As a shorthand its France vs America. You'll note that we're not using Minitel bur rather a board hosted by a VC operation who shares a mindset with the founding team of Uber.
Hazlitt: “The art of economics consists in looking not merely at the immediate but at the longer effects of any act or policy; it consists in tracing the consequences of that policy not merely for one group but for all groups.”