One extra trade route will add roughly 6 (raw) per turn. Most cities have around +75% commerce multipliers (some much more), so we're looking at +10/ per turn from the Airport. At 250, payback will be therefore 25 turns (as compared to 'build wealth'). Better cities with more multipliers or better trade route yield (as noted, the capital for instance) will get more.
As for war, I don't expect anything too worrying either, but I'm nonetheless wary of the possibility of getting backstabbed. Maybe in a single player game I'd be happy to have coastal cities guarded by an Archer with no meaningful reinforcements in sight, but here I'd pull back some forces and make other preparations.
We will not need 25(28) turns of research. Also a large city will steal a profitable trade route from other cities so it will likely not be 6 commerce bonus. Probably we can get a trade route at 6 commerce. But then a 5 commerce trade route in a smaller city was replaced with a 4 commerce one.
Question should be if we want to avoid artillery and get it for free?