In this post, Roko makes some simple estimates to conclude that there are not really any physical/material constraints on humanities' growth until about 10 trillion people.
Personal rumbling
That paints a different view of the future than I had in mind intuitively. Perhaps humanity could mostly solve resource scarcity within ~50-100 years and allow for rapid population growth (say, enforced by local governments for some reason [like trying to game global democracy, if I'm already painting one specific and weird future]). This type of thinking might put a big value on the not-so-far future, which potentially could be addressed by interventions other than advocated for in longtermism (say, advocate for medium-term (~100 years) contracts/policies, something about global warming, improved population policies).
More concerning than jet engines might be the high efficiency natural gas turbines for generating electricity. However, it looks like Ruthenium is even better than Rhenium for these applications. And in general, you can avoid rare earth metals and just accept slightly lower performance for combustion turbines, wind turbines, electric car motors, LED lights, solar cells, etc.