I disagree.
Technology is the only option besides euthanasia or actually killing people in a regular basis - and I doubt very much we’d like any of the latter options. Cosmic radiation is solvable and I never said it’s Mars we need.
Apart from that: The planet is and will be habitable for quite some time - but we’re going back to square one and the question will be: Euthanasia or outright killing those that have no say.
Probably around 80 million, give or take