If there's some message the filmmakers are trying to push it would seem to be that things can become desperate but don't do the "bad" choice when things become desperate. It won't work out. And the heroes take the everyone is important stance, which, since they're the heroes they have the "good" label and Thanos takes the some people are expendable stance, and since he was promoted as the villain that gets the "bad" label and the choices that align with that stance are the "bad" ones. And Thanos himself is desperate. He's more long term desperate but it relates to the immediate desperation of the heroes.
I think the root of Steve's conflict of theory and action is his view of himself. It's also what supports his optimistic viewpoint. He was the little guy, that during the time of war still wasn't found to be useful, but then things turned around and now he's a super hero helping people. So every little guy deserves a chance. But deep down he's still the little guy and any positive impact he had is a bonus. And if he were to sacrifice himself that's still an overall net gain.
I agree that the Russo's definitely take their movies in a different direction and I find it refreshing.