Yes, you are right. As it stands right now, upzones are valuable because high density land is scarce, upzones are rare, and the price difference between higher density zoned land and lower density zoned land is so drastic. A much larger scale upzone would tank the value of high density zoned land to a much lower equilibrium.

The more important question is why we should settle for policy based on the economic rationality of people whose interests hurt society? Should we cave to the Koch Brothers too?