Oh I agree. Every house now is a potential development property. But everyone keeping their new price high means not every property will be developed.
And yes, this does not really help affordability.
Ask Helps to point to a jurisdiction that did this and show how affordability improved. Never happened in the history of the world. But won’t stop idealogues.
Alternatively if you do conditional rezoning (ie: if there aren't any other similarly developed properties on the block, the area has the capacity for increased density AND its consistent with OCP) - you create an incentive to be "first mover" as once the one parcel is developed, there isn't another parcel zoned for it (second movers are too little, too late). A developer or more ideally a co-op would approach all owners on the block, and the one who offered the best deal would see their property redeveloped, as it is competitive, there's a better chance of the developer getting a reasonable deal on the land. There's also more incentive to "go first" so in terms of getting development done quickly, it's better as there's incentive to do it sooner rather than being unable to do it at all. As a bonus, it maintains neighbourhood character and prevents entire neighbourhoods from being remade as the zoning isn't blanket in nature. It would also help distribute the density to areas that are below target density, while preventing areas that already have higher densities from becoming overwhelmed. One house on a block being made a town house is likely no big deal, but if all the houses were, many would feel that the nature of their neighbourhood had fundamentally changed.