It's called supply and demand. It's called life. Things change. The world doesn't revolve around families that "shouldn't be uprooted and have their lives thrown into disorder".
If those families wanted to be able to stay there permanently at a fixed rate of housing expenses, they should have purchased the property. Can't afford to buy? Then deal with the risks of renting.
Their desire for low rent that keeps them comfy and entails no inconveniences has no validity. That's just a wish. That's immature. This is not grade school and not everyone gets a trophy for simply showing up.
The problem is not the new people moving in and the solution is not to stop them, the solution is to address the pressure those new residents create that is driving the old residents out. Entire families shouldn't be uprooted and have their lives thrown into disorder just because some twenty-something wants to live in SF for a year or two while they're working at a startup, and that twenty-something shouldn't have to practically evict some low-income family to find an apartment.