If the Karlsson deal ends up not working out, the Penguins will still need to add an offensive threat from their blue line and I think Faulk can make some sense. He's 3rd in the NHL in 5v5 goals and 4th in the NHL in 5v5 points among defensemen in the last 2 years, so he'd be adding a significant offensive threat from the blue line.
I was wondering what the Penguins would have to add on top of Petry to pull off Faulk. Faulk is definitely more valuable, but I'm honestly struggling to guess what his value would be right now.
What's in it for the Blues? This should be a mandatory question on this forum. If you propose a trade, you need to at least try to come up with a reason for the other team to accept it. You list all the reasons Faulk is better and why Pittsburgh would want that upgrade. Why does St. Louis want that downgrade?
If we are rebuilding we don't want the older, worse D man unless you value him as a pure cap dump. He has no value to a rebuilding team as anything but that. If we are retooling, we don't want the older worse D man whose contract would end right when a retool would be complete. So again, he wouldn't have any value. If we are competing, we don't want the older worse D man period.
So the question isn't what would you have to add to Petry. The question is, if you are rebuilding/retooling, then what is the value of Faulk in futures and could you take Petry to make cap work? The answer to that would be a near-ready defensive prospect with top pair or #3D upside plus more. I am not familiar with Pittsburghs pipeline, but I don't think you have that prospect. But that is the only way trading Faulk makes sense for the Blues as a future play. We get worse in the very short term to get younger.