I think in isolation, allowing construction of a single apartment building in an other SFH neighborhood might push down nearby property values compared to a sub

I think in isolation, allowing construction of a single apartment building in an other SFH neighborhood might push down nearby property values compared to a suburb without the apartment, particularly in places where apartments are stigmatized as being "too urban" You are quite correct that upzoning everywhere only boosts the value of land by allowing better utilization of it. One possible exception might be that by solving the housing crisis by building enough units to satisfy the regions need, it might push down the value of land, especially in the less desirable periphery where you might have previously had folks paying a lot of money just to get a house somewhere.