There's zero chance that anything being discussed in here would relegate Backes to a 4th line role.
I can easily see an argument for Tatar being better than Brouwer, assuming their usage would play to their respective strengths, but there is no real reason to believe that Svechnikov is NHL ready (much less that he makes anyone one the current roster expendable). Tatar would also likely cost more than Brouwer would once he signs his next contract, so it's not like that upgrade comes with a corresponding cost savings beyond the first year.
If the intention is to use Shattenkirk's trade value to replace one of Backes or Brouwer, then there needs to be a better player coming back...someone that's a cut above what we might easily acquire via UFA and/or the player(s) we're letting go, or alternatively someone that's similar talent but at a significant long-term cost savings.
If the Blues can't manage either one of those things and they're still committed to trading Shattenkirk, I'd rather just see them trade him for the best futures package that is being offered.
The Shattenkirk deal is a tough one to come up with. Is there a fit?
People assume he will sign in the east, I can't find any proof of that.
One team is Dallas. Goligoski, Demers, Russell are all UFA so they need a defenseman. Shattenkirk fits into their offensive style. But what is Dallas willing to give? Nobody that I can think of that we'd like.
I'd settle for future pieces depending on the those pieces. Some have proposed is a 3 way which ends with us picking #4. Obviously the details within the deal haven't been set so what would those be? The other two teams involved are Edmonton and New York Islanders.
Rangers fans have obnoxiously suggested Rick Nash but that ain't happening. Rangers have no future pieces and none that they'd give up. No one else on there roster of interest.
Boston is a possibility but Krejci has a NMC. That leaves Spooner. Who, doesn't seem like he'd be a great fit and probably never a 1C. So nothing there.
So to come up with a logical idea for Shattenkirk would need to establish all teams with interest. Regardless if whether he'd resign. Then establish value (resigning would weigh in). Then establish which players give that value.
Is Armstrong interested in future peices? Players to help now? Potential 1C (upgrade on Lehtera)?