Bergevin hasn't addressed our real needs and instead focused on solving locker room and attitude issues while paying crazy cost in order to do that. But, the organization he inherited was already suffering from the symptoms of poor management. The prospect pool was poor due to bad drafting, there were large holes in the team and the development staff was incompetent. Bergevin didn't necessarily destroy the team, it was already doomed barring major changes, but Bergevin does deserve a lot of blame for not making those major changes and instead deciding to fix absolutely tertiary issues (which also failed miserably). Just to make it clear.
Subban into Weber=huge loss
Sergachev into Drouin=huge loss
Price contract=awful and turns a top 5-10 goalie into a negative asset
Radulov and Markov walk for nothing=so so stupid
Tank Patches value by surrounding him with trash=huge loss
Trade Andrighetto for nothing=pretty big loss
Galchenyuk somehow went from a great 3rd/4th year looking like he had ppg potential into being demoted to the 4th line and not getting C roles despite the team's C depth being the worst in the league and then was traded for Max Domi=total stupidity
The only person worse than Bergevin is Molson for not firing him, he has destroyed the team and maybe the team would've never won a championship with their core, but you can always tweak things or at least get fair value for players, Bergevin has failed and failed badly.