This is a very uncharitable take. People choose a place to live based on criteria they like. If you want to live in a super dense city you might gravitate to Manhattan. If you want to live in a quiet suburb you end up in such a place.
If suddenly that reality is torn down and substituted for something you didn't want, it's going to hurt. If you wanted a quiet suburb and you found it, you won't be happy if suddenly all your neighbor houses are torn down and replaced with apartment buildings. It has nothing to do with "not wanting undesirables". It is because the conditions they wanted and bought into are no longer there.
> from a financial sense, upzoning is a big positive for existing landowners
Except for house flippers, people don't care about the financial sense. A house is a home, in a place they like. Telling someone that the neighborhood they cared about it no longer a reality but it's ok because they made money, will not satisfy anyone. It's not about money.