If someone is living in the mission there are only 3 options; 1) They own their house 2) They are renting but their unit is rent controlled 3) They are rich or have high paying jobs (and building more actually helps them)

"Because the actual building they live in gets demolished to build another building"

This happens very rarely, people will literally revolt before a rent controlled unit gets demolished in the mission

So someone opposing more building in the mission falls into the first two categories and they are extremely selfish