Being better than Burns defensively at the same age is, as you said, not an accomplishment. Rangers fans for years have suggested converting Miller to forward because of poor defense, only to ultimately reject the idea because he’s not very good with the puck either. Burns was converted to forward briefly because he was THAT good offensively.
Burns was an elite defenseman because of his offense. Miller doesn’t have Burns’ shot, vision, hands, physicality, or puck-carrying ability—basically not a single one of Burns’ calling cards. If you’re arguing that Miller might have things just “click” for him at age 30 like Burns did, you’re fighting big odds. The vastly more likely scenario is that Miller continues to just be a guy who’s big (but not remotely physical) and skates well (but doesn’t do much with it). Decent middle-pairing defenseman.
We shouldn’t be spending assets on guys who don’t have a long-term future here unless they’re great development fits. Miller isn’t that. If I thought he was a legit #2 defenseman who could do spot duty as a #1 and not look awful (which is what Burns was when we acquired him), then I would see your point. But I don’t think, nor does anyone else I’ve seen think, that Miller is that guy.