Downside to losing Petro is obvious. He's a top 10, if not top 5 defenseman in the league. He will not be replaced by Faulk or Parayko or anyone. You just hope the D will be collectively good enough to overcome his absence. I think that might be tough. The upside it that it frees up cap space for a few good signings or to keep young players they need to keep.
One thing many sports fans don't seem to get is that five really good players on your team is not as beneficial as having one great player, most of the time. The AV's have Nate MacKinnon and other good players. Other than O'Reilly and 3 players, there are not any 3 or 4 players on the Blues I wouldn't package up for one Nate MacKinnon (or a Crosby in his prime or Ovechkin in his prime, etc...). The closest thing the Blues have to a superstar is Petro and O'Reilly. Other than those two, we're looking at some very good talent but not great players. A few could get there, specifically Rob Thomas but none are there yet.
I also think Tarasenko is done. I hope I'm wrong!! A hockey player that plays physical and has a big shot like him, can't be successful with a bum shoulder. To say his shoulder is a little banged up is like saying the Titanic was taking on a little water. He has a major problem and I doubt he will ever be the player he was... On the bright side, he was pretty instrumental in bringing a Cup to STL!!
-------------------- "Hunts are best measured by the endurance of the memories they produce..."