There is no rational link between a barrel bomb containing noxious chemicals and a missile containing a VX nebuliser over a city of your choice. Thermobaric bombs are equally bad - people shredded with overpressure.

Phosgene is degraded by water, especially if alkaline. Pretty easy to remove- and that is a really old one. I am sure newer ones have a very short half life.

I would hope that our politicians are above the Id of the general public - chemical weapons have been recently used to assassinate people both in Asia and Europe and that was pretty much ignored and the agents used were far nastier. Before that was the Polonium assassination.