There is no way to know the actual results of either action or inaction.
And the same will be true if Assad continues to use lethal gases in spite of punishment.

Regardless of treaty obligations, any government with WMDs may be expected to use them if they think their backs are to the wall and their personal safety is not at risk. That is simply a statement of human nature. The only way to force Assad and his crew to relent would be to nuke them. Personally. That option is not on the table. Nor should it be.

Maybe few missiles will influence the man. Maybe not.

The US could save itself a lot of money by flipping a coin. Regardless of the ultimate decision, more complicated methods in this case offer no better promise of a good result.