I find it's pretty much impossible to engage with people who still think Ukraine can either win or negotiate better terms by prolonging the war. No matter what you say, they're just going to counter with that.

It's going to be easier to talk about this once the war is over and Russia has won. At that point, they will have to grapple with the fact that Ukraine is objectively worse off than it would've been had it taken the deal two months into the war. Once that's established, then it's possible to start talk about whether the west actually helped Ukraine in the end.