A substantial amendment to the Business and Professional code could defacto overturn it.
That isn't the only way the props effects could be effectively bypassed, they could be bypassed by other updates to california law, e.g. to unionize app workers, changes to laws governing working conditions for drivers, etc. All such changes are potentially pre-empted by prop22's language.
Prop22 arguably creates a fairly broad automatic veto for Uber/et. al. against any California law that could be argued to impede the rideshare business. For example, there is a clear argument post prop22 that a law which prohibited registered sex offenders from being rideshare app drivers would now require a 7/8th's vote.
Uber has repeatability shown itself more than willing to exploit loopholes in an unethical manner and we've now granted them a fairly generalized shield against loophole closing.
Ultimately the courts may not be willing to follow such a broad effect, but the recipe for a lot of time and money to be wasted litigating it is coded right into the proposition.
