What do you mean? Saad clicked great with Kadri and Burakovsky. That was a very good line. They were just trying to balance out the scoring, and didn't want to break up Naz and Burky.
And why are we saying he's underperforming? He's on pace for 30 goals and the same 50 points he gets every year. And he's coming off a stretch where he had 10 points in 9 games.
Nuke has replaced him very well though. At his age, with his better defense, and better forechecking, he'd be the better signing if he comes in around the same price, and can continue producing around the same 20+ goals and 40-50 points.
I recall that the 2nd line of Saad-Kadri-Burakovsky had long stretches where they weren't producing, and there was accordingly plenty of discussion on here about that in gameday threads. That isn't to say that they didn't have their moments, because they did, and some of the inconsistency was down to Kadri under-performing, but ultimately Bednar clearly saw fit to reduce Saad's minutes and get him off Kadri and Burakovsky's line. See the TOI chart below for reference, where it seems that Saad's role kept diminishing over time in the regular season.
As for the reasons for that, I disagree that the aim was to balance scoring. Donskoi was doing great next to Nichuskin and Jost (again, that line was 1st in the league in expected goals for/against), so if the Saad-Kadri-Burakovsky line had also been as functional as you're saying there would have been no reason whatsoever to break up either line, but Bednar did.
For the record I didn't say that Saad was or is under-performing. He clearly didn't given that he paced for ~45 points last year, and is pacing ~50 points this year. He was undoubtedly helped by an unsustainable shooting % last year though (22.1% in the regular season and 36.8% in the playoffs). In my eyes when reflecting back I see a player who: didn't find great chemistry on our 2nd line; who's coach reduced his minutes/role; and who had a crazy high shot% in a contract year, all of which played a role in why Sakic didn't even try to re-sign him even at a seemingly good caphit.
Now with all that said, would I prefer if he was on this team at $4.5m instead of Compher at $3.5m, yes I would. His ability to score dirty goals and play a dependable two-way game would have been a nice fit on the 3rd line next to Newhook this year. But in a vacuum paying $4.5m for a guy to play on your 3rd line wouldn't have been very wise, especially given that he likely would have seen a reduction in production in the same role when his shot% returned to normal.