Why?

You say extinction is a very real possibility. How? Please explain. If it is directly related to habitable zones, explain why nothing will be a habitable zone. If it is indirect, please explain the mechanism.

Why are you even mentioning immigration?! Existential threats include things like very-high-mortality-rate highly-transmissible viruses, all-out global thermonuclear war with large stockpiles being utilized, several-kilometer-or-larger impactors, the Yellowstone caldera erupting, global loss of control of an automated military (if we build one), etc..