If people actually supported preserving existing units/communities exactly as they are, with no change, then they would vote to expand rent-control to 100% of a

If people actually supported preserving existing units/communities exactly as they are, with no change, then they would vote to expand rent-control to 100% of all rented units in the city, thus ensuring that anyone who gets an apartment in San Francisco will be able to stay indefinitely. But actually, people only care about their cheap housing, not cheap housing for everyone.