Well, true, the credibility is lacking.
However, let's look at what happened. We know that there has been more than a little fuckery in Syria. We like to say there are good and bad guys, but the fact is that there are so many different hands in that pot, an entire field of study could be dedicated to figuring out who was cheating whom.
With that said, the strikes took out capability that was being used by various people. Maybe there has been a lot of misartribution, but the toys have been taken away.
If attacks happen in the future, then someone had to give Assad - or whomever is blamed - the weapons. Further, this has become a major "Trump is an evil-bad for having stock in the missiles" thing among the rabids. As this begins to prosecute and settle out (or spin up….) There is the possibility that major military action and risk of war was executed based on manufactured evidence.
But I risk interrupting their mistake to go further. Let us just say that the media only investigates when they think they have some dirt on Trump. And they are compelled to do so with the force of a million quazars. Hell, I could probably outline exactly what they will do and why it will be their own undoing… And they will still do it, with even more savage vigor than before.