“For example, if it comes to light that we can’t prove it was Assad”.
I would agree with you. in THAT instance, that Trump will have less egg on his face, but “western intelligence”, particularly CIA and State Department (more than DOD), and even some foreign services, as in France and U.K., will have considerable egg on their faces - AGAIN - and that will raise the bar higher the next time they have concerted critical claims to make. THAT too, though, given it’s possibility, is part of the reason Trump should have said nothing. It is not just his own reputation he put on the line, but that of intelligence officials reporting to him.
He should have (1) said our intelligence community is assessing all evidence, (2) said nothing about any possible U.S. response UNTIL, or maybe not even after, all U.S. intelligence assessments were completed and ONLY a consensus of a determination be given voice as to policy, and then (3) that consensus must agree the evidence is not just possibly but definitely pointing at Assad et al, or that it is insufficient, or insufficiently agreed, that it points to Assad. (4) Then only if the consensus agreed the evidence pointed to Assad might Trump take action, but still in my view it would not be action he would announce ahead of time.
It can tell us where Russia is embedded and thus reveal where Syria's key military equipment (command and control) is located. We have probably x satellites and x drones watching it all.