Sorry, but not when the 2BR across the hall is renting for $4,000/mo and is on the market for 4 hours before someone gets it. Grandma doesn't have a right to live in the exact same apartment for the rest of her life at the expense of everyone else.
I'd be OK w/it if you have a humane way to implement it but I doubt voters in the city would ever sign off on it.
I believe the landlord is bound by the language of the law, not the contract. If the law enabling grandma's rent control were fully repealed, there'd be no rent control-related reason the landlord couldn't raise Grandma's rent to market rent. Basically, Grandma AND the landlord both needed to know what they were getting in to.
Grandma: "I want this apartment."
Landlord: "OK, it's $250/month with rent control. But just so you know, if they ever repeal that law, and market rate is more than the $250/month you're paying, I'm raising you rent."
Grandma: "Understood. If I were the landlord, I'd raise the rent on the grandma renting my apartment, too."
In California, this isn't a hypothetical. Especially because she pays property taxes as if the thing is worth a number much closer to $20k than $20mil.