Byram apparently wanted a trade so he could be on a team where he plays a bigger role. Considering Toews and Makar are going to be the top pairing for the next five years at least, that's fair enough. I saw a clip of a Sabres powerplay where Byram carried the puck into the zone and was trying to tell his teammates to move around and actually do things, so he might have bitten off more than he can chew.
For various reasons, Byram's Avalanche career always felt like you were just waiting and willing him to do more. Injuries cost him a lot in his early years, and they were concussions so when he came back you cringed every time he had contact. He also started on the team when the schedule was disrupted because of Covid so where was never really a sense of normality in his playing time. It was obvious that he was a total baller and the best sort of sportsperson - the one who's at their best when they're allowed to do whatever they want. He's not going to get the time to do that on a team with Makar on it, so there was always a sense he was never going to properly fulfil that potential.
That said, I don't think player usage here helped. Makar is the only D on the first powerplay unit. The second unit had Toews, Girard and Byram on it. The first unit basically stays out for the full two minutes. If Makar wasn't playing then Toews would take his place on the top unit. It's always going to be difficult to break into a defensive group as deep as ours, but there were definitely times when Byram could and should have played more.
Mittelstadt is a player in a position we needed. He's looked alright.