A. If the "free-er market" (allowing 4-plexes within building envelopes) is the better, more natural market state- that means the market under exclusionary zoning featured prices artificially inflated by government intervention. Should those gains have been "socialized"?
B. What's a real loss? A paper equity drop of a few % points on the unrealized gain of an asset that's been held for decades and 10x'd surely is not.
C. Why are you so convinced that there will be losses? Either you're right and the real market shortfall is for big homes and big lots and the zoning discussion is much ado about nothing (as the buyers for those homes/lots will be willing to outbid those that want to develop) and the neighbourhoods will remain the same, or the developers profit motive will see them outbid the motivated buyers and the value will actually go up.