Blues never needed to burn anything down, they had a good mix of youth and old and if anything Berube was likely not doing them any favours in his system and how much they were underperforming for what they had on their roster, it was very apparent when Berube was fired where most of that team was struggling and how improved they were without having to make sweeping changes to the roster.
I think the hype for Berube as a coach is kind of funny because of that. They were 1st round exits and then went as far as the 2nd round before the following season they were playing like absolute crap because Berube's system really was just caving them in, Bannister taking basically that same roster and having them just barely miss is a great example of why Berube needed to go asap. I mean quite literally - The Blues didn't trade for anyone last season when Bannister took over, he took over the same roster and put up 30-19-0-5 vs Berube's 13-14-0-1.
Under Berube last season: 13-14-0-1, 79GF, 93GA (-14 differential), 2.82GF/G, 3.32GA/G, 31st ranked PP, 20th ranked PK
Under Bannister last season: 30-19-0-5, 155GF, 155GA (0 differential), 2.87GF/G, 2.87GA/G, 13th ranked PP, 13th ranked PK
They definitely struggled to score but that could also be because their depth scoring was still crap while their better players started to produce more which had them start scoring at a better rate but just barely, but defensively they were better and so was their goaltending benefitting from that.
Thomas, Kyrou, Neighbours, Krug, Saad, Faulk - All improved under Bannister, the slower players didn't really do much better and some of the older players definitely didn't fare well under Bannister as more youth were played instead (must be nice). Hayes, Sunqvist, etc. If you look at who Armstrong brought in it also supports that - Texier, Holloway, and Joseph are all better skaters. Faksa they picked up to replace Hayes in the bottom 6. They added more young players (aged 27, 22, 23, and 24) while adding 2 older veterans like Faksa 30 and Suter 39. Even with adding Suter and his almost 40yr old ass, they're still younger than the Penguins by a decent amount.
So really, sure you might not need to blow it up, but the Blues acted quickly and fired Berube before it was back to back years of missing the playoffs and more damage was done, they hired Bannister who who like Berube before him when he took over a fired coach, was able to turn around the same roster and do a better job. Penguins have let Sullivan stay so long that the problem might not be fixed with just a coaching change, it'll maybe take more because for almost a decade this team has drafted, signed, and traded for "his type of players" and there in a pretty shit spot because of most of that. So the first step would be firing Sullivan to see if a new coach can deprogram them, if that doesn't work then yeah - very big changes.