The more I think about this based on the moves the Blues have made, the more the obvious answer is Berglund. Look at the Blues signings so far. Top line center, Bottom 6 center, depth center. Berglund slots in as the team's 3rd line center behind Backes and Roy now. The Blues have other options there, and given the Roy signing, it seems like the team may move away from the "big center" philosophy a bit. Sobotka could easily manage 3rd line center duty. Sure you would miss Berglund's offensive output in 30 games out of the season where he shows up and scores 20 goals. The other 50 games, you won't even notice Berglund being gone. If there are injuries in the top 6 at center, the Blues have Schwartz and Steen who are certainly capable of doing the job.
Also, there is the matter of contracts. Berglund is on a 1 year deal set to expire at the end of the season when the Blues will have to shell out at least 10m dollars to keep Steen and Bouwmeester. Looking at some of the forward contracts being handed out this offseason, he will probably be looking at a contract worth more than Oshie's or Perron's which IMO isn't worth the investment.
Finally, people say Perron has to go because he can't play for Hitchcock. I seem to recall Bergund being bounced down to the 4th line on more than one occasion last season.