Interesting that Tesla is pivoting away from cars. Are there going to be replacement models or a complete exit?

The projections of peak population in 2080 which you refer to are from the UN - they are outdated and incorrect. Indeed the UN has been consistently forecasting many low fertility countries to have substantial implausible increases in fertility. This has obviously not happened.

Based on the latest figures, which include huge drops in birthrates in the last few years, the total population peak will be in 2060 or even earlier.

But it is peak working age population, rather than total, that is most relevant economically.
This may peak as soon as 2035.

Demographers I follow, using latest figures, say Global fertility has dropped below replacement already. African birthrates are dropping sharply & almost everywhere else has fallen far below replacement.

The thing is that Making Life Multi-planetary needs (1 million+) relatively young (fertile) people to move to Mars and have plenty of kids there.

Thus Musk is really racing against time. Even assuming (very) optimistically that mass settlement of Mars could begin in the 2034 launch window, he will be seeking settlers from a fast shrinking pool.