"If the gassing of the population didn't have any dramatic effect..."  This precisely the point.  The dramatic effect produced was an attack by Trump on Syrian government forces.  The Syrian government had reasons not to use CW rather than to use them.  If an entire rebel held town had been wiped out, one might concede that government forces would do it to avoid street-to-street fighting, i.e. a military benefit that would save some of its soldier's lives.  That is not what happened.  A small number (in the overall context) of people were apparently killed by chemicals.  Use of CW to cause this small pocket of deaths provided negligible military advantage but did provide dramatic effect, a high probability dramatic effect, of opening direct conflict between the Syrian government and the US.  In the end, Assad cannot survive US involvement; therefore, out of his self interest, he would not use CW.  The rebels win big through their use.