Earlier quoted context omitted.
This is a reasonable question to ask, although it's also a terribly wrong assumption. First the ship has already sailed for most of Asia, where the fertility rate is already below 2 or near 2, and the billions extra are just from people getting older. China's population for instance is already expected to decrease from 2030-2035. Asking from these countries to reduce their population is equivalent to asking them to r…
Of course it is the right question to ask. If 100 million people fly and contribute 2.5% of emissions, and 7 billion people contribute 25% of the omissions are using coal generated electricity, which probably should you solve?
2) The 100 million people who fly have the most concentrated wealth and therefore have more leeway to implement alternatives to flying (high-speed rail for flights within the US for example).
3) You could also imagine the 100 million people investing to build new low-carbon electricity generation infrastructures in developing countries via a carbon tax on their flight ticket. Of course, they could donate directly as well, either solution is fine. That was actually the spirit of the Paris agreement, which Trump walked out of. Since the 100 million people who fly actively refuse to let go of their wealth, alternative options such as 2) are being considered, even though they might have lower impact (since they affect on the 2.5% of GHG emissions instead of the 25% of GHG emissions).