This itself seems like an imprecise argument. If there are well-known laws against doing something that has gone unpunished so far, is it then legal?
If you can trespass for 20 years (varies by jurisdiction) without getting ejected, it's legal. And you now own the property.
If you don't enforce a trademark, it is void. There are statutes of limitations on a lot of crimes and torts, including those involving violence. So there is quite a lot of precedent involved in unenforced, flagrantly-violated laws becoming void.
One way around that is to elect government attorneys that will prosecute those laws, and government cops that will investigate suspected violations of those laws.