That is assuming the colony actually is reasonably self sustainable, which is quite a high bar.

>The question is could you save more people in space in a hypothetical end of world scenario than you could people on Earth dying in a very real scenario today.

There is no certainty of an end-of-world scenario but there's also no certainty that a cure for cancer actually exists or would be found. There is reason to believe and enough supporting evidence that a space colony would be theoretically possible - even if not self-sustaining for quite some time. So the question is a very real fix to a theoretical problem vs a theoretical fix to a very real problem.

I don't want to divert money from cancer and presumably other medical research as well, so that the last N humans can live in a space capsule. Mortality rates for cancer have been on the decline for decades, so the issue is not theoretical.