It's not a government problem - it's a constitutional one - Banning existing billboards is considered both a first amendment problem, and a takings problem.
Doing it without compensation is a constitutional violation, and even if california modified it's constitution, it would be a federal one. Less of one for sure, but still an issue.
The current law (Business & Professions Code § 5412) is a codification of what the court requires.
>Doing it without compensation is a constitutional violation, and even if california modified it's constitution, it would be a federal one.
Only matters if the constitution is enforced. Courts can easily overturn previous decisions. If there's a will, there's a way.