Why is the burden of proof being changed in public discourse about the chemical attack? Why should there be a defence against the credibility of the attack in the first place when there was little proof to begin with? By this point, is there anyone that really believes that this wasn't the product of the White Helmets?

If there are allegations of the chemical attack being legitimate - then it is up to the US, UK and France to provide the necessary proof. THAT is how the burden of proof works; it lies with the claimant.