The idea that some astronauts would survive doesn't make me feel like condemning myself and billions of other people to death. But, I can't speak for everyone.

And I certainly agree with you; however, the "Plan B" narrative is strong - strong enough to potentially mobilize significant financial and engineering resources toward the cause. When the narrative states that "man must become a multi-planetary species" very few people are thinking that those extraterrestrial environments will be spartan, hostile, and (possibly) reliant on earth resources. They imagine Earth v2 - but better because science (fiction).

If we're need the public to change their behavior to preserve the habitability of this planet, then giving them excuses for inaction may not be productive.

It could also be a great live demonstration how nearly impossible it is to live without a functioning Earth-ecosystem around us. Especially if it doesn't work out exactly as planned.

Until we bioengineer better suited humans.