Sure, in this scenario the "protecting their property value" person would just cash out and sell to the developer who's gonna be good with tenants and is willing to pay more since it's no longer single-family-restricted.

But the opposition to upzoning here in LA and the Bay Area is largely coming for qualitative reasons instead of quantitative reasons. And that's lost when you reduce your argument to "they only care about their property values" so you lose any chance to convince them...