With global population declining how are we going to sustain a colony on Mars?

It's going to take a decade to set up a lunar research base right now.

Safely assume another 20 years to set up a Mars base.

The US has a below replacement fertility rate so you are going to have to send more and more people to replace the population on Mars.

With the world population declining increase in Mars population will be due to decrease in the population of Earth.

This whole system only makes sense if the most advanced society in the world is on Mars ( Only the smartest people went to Mars ), and the poorest regions of the world are emptying out as their people move up the ladder with the most backward regions on Earth being taken back by nature.

None of this works until and unless nation states reach a point where they are again up to replacement fertility levels.

Developed economies do not have above replacement fertility rates.

They are not in a position to send their populations to colonize another planet. It's not a tech issue, it's a manpower issue.

Even South America is below replacement.

Oceania will be below replacement by the end of the decade, and only Africa is going to be above replacement levels.

African IQ on average is too low to send Africans to run a space colony.

In conclusion:

We could build a Mars colony, but we couldn't fill it.