The standard leases used for rent-controlled apartments are very explicit about this, so unless a landlord used a non-standard lease, or didn't use a lease, this isn't a real problem. You don't have to be a smart landlord, you just have to not be a stupid one.

There's no issues around discrimination for this, and it's not an uphill battle. Replacement tenants are not co-tenants, and do not have rent control protection if the original tenants move out. Landlords cannot reject replacement tenants in most cases, but they have no requirements on accepting replacements as co-tenants. Replacement tenants are added as sub-letters of the original tenant.

I was a real estate agent who worked in property management, and I've also lived in rent controlled apartments in SF. You're absolutely wrong here.