I don't really think about this much, but your comment made me wonder:

If we do find another earth-like planet within travel distance (impossible afaik but let's suspend disbelief for a moment), how do we determine whether it's worth colonising? And how to we measure it?

"The resources on this planet will last 15.6B person-years which means if we send 5 million people there over time, we will have to prepare for their evacuation in ??? years"?

Obviously totally moot if Earth's resources aren't going to last that long, but just had that thought bubble up.

The "bigger problem" is that it is insufficient to observe the life carrying capacity of a planet for a few decades and conclude that it is stable long term.

For example, the host star could have variability measured in thousands or millions of years that would render the planet inhospitable to humans but not the indigenous life, which would have been adapted to these cycles.

Similarly, the planet could experience regular asteroid impacts due to passing through a recently broken up rock that intersects its orbital path.

Some of these risks can be eliminated through careful study, but this would require something like a century of painstaking geology, thorough astronomic surveys of its neighbourhood, a full fossil record, etc...