Fortunately, it's also its own solution: if we find we need to avert a catastrophe next week, grounding all flights today might do the trick. But of course it's very hard to predict a tipping point to that level of accuracy. If we had that sort of movie-plot threat in climate change we might actually see meaningful action.
So yes, while carbon dioxide and methane are terrible, awful, no good gases we shouldn't be dumping into the atmosphere with quite such abandon, choosing to maintain water vapour at a particularly vulnerable point in the system is also something of an unforced error.
I feel unforced error is bit harsh too. Climate sustainability is a series of mostly hard problems, and aviation is one of the hard ones.
Perhaps there is an opportunity for a fuel additive that minimizes the formation of cirrus clouds, if the clouds are the source of so much of the climate impact.
Excess H2O (water vapor) falls out of the sky as rain, so it's fairly constant (ie. "maxed out"), and its long term concentration levels are a function of CO2 equivalent gases in the atmosphere. So if you increase CO2 levels, H20 levels will naturally increase as well, and vice versa.