Mankind will definitely not survive long term trapped on a single planet. So this malthusian thinking is counterproductive. The universe is infinite and it makes sense to think not about how to fare a bit longer on the bottom of a gravitational well, but about how to conquer new frontiers.

We aren't going to run out of some essential resource - as they will get more scarce, the prices would increase stimulating the search for new sources of that resource or its replacement as well as more careful spending of the resource. (Read Julian Simon).

Also, it seems that we are going to run out of people first - the demographic is collapsing, at least for more developed nations, and the rest is soon to follow.

Also, speaking of food for example, the demand is not so elastic, we may eat better or more fancy food if we can afford it but there's a limit after which we can't increase our consumption here.
There are a couple of things worth caring about though: species and civilizational knowledge, which are hard to recreate if lost.