Blues have great development and great pro and minors scouting. Not super surprised that a young pick up has worked out well for them, Armstrong is a great GM, I've seen it all before with his brother
Broberg wasn't given much of a chance on the Oilers and was generally decent or better for them, but he barely played for the team over 3 years, totaling around 80 games + playoffs over that period of time. There was never the kind of trust that was needed for him to play big minutes and get established as a d-man in the org because Ekholm was already sheltering Bouchard and the rest of the Oilers squad didn't have anyone who could shelter him. He absolutely could have been played over Desharnais but there was practically no room for him before last year with Barrie, and the year before Barrie and Keith were both there which left even more of a logjam in a team that was trying to win real hard. I can understand not wanting to trust a 21-23 year old defenseman who is primarily an offensive player and hasn't really improved his defense a ton when you already have Bouchard and Nurse on the left side.
And Kulak isn't an amazing defenseman by any means but IMO he's better than Broberg in many aspects of the game and Broberg was never given the opportunity to shine and was asked to play a game that he wasn't super fit for as a still developing prospect. Oilers heavily needed players who could handle guys in front of the net and in the corners, guys who could clear the crease and handle players one on one on rushes. Broberg is a great offensive defenseman and has great stick work and skating but despite his size he's not a guy who throws around his weight a ton or is particularly strong or mean.
It makes perfect sense what the Oilers did, not gonna speculate on whether it was the right choice or not but they got a 1st, 2nd, and 3rd round pick for players who they probably still weren't going to give significant time to while in their cup window.
I don't think this is really on the GM at all, and is mostly on the coach if they do pan out. Although Broberg really hasn't developed a ton (don't get me wrong he's a good player but he's still almost the same player he was when he was drafted) and Holloway hasn't been lighting it up for the Blues at all, but while I'm sure the Blues are happy it's still only been 5 games into the season.
By far not useful to compare it to shit like Erat and Forsberg at this point. Dvorak and Strome were a PPG when they came to Arizona at first too. There was a season when Brad Richardson had a career high and team high 19g. Sample size is there for a reason. That Arizona team was hot f***ing garbage btw but they still had the 3rd most SHG of any team in the modern era with 16 (behind 2007 Blackhawks and 2023 Oilers).