"Every time it goes in, ka-ching $$" -Tkachuk.
That S% was never sustainable and he never drove his own line, even at his best. Why they didn't go bridge deal I'll never understand. There was countless threads online advocating for this and many Sens fans I spoke with weren't sold on him. Gave him the bag to ensure Tkachuk's bff is happy. Soft.
And if he had gotten a bridge deal, the injury didn't happen, and he continued on his trajectory, someone just like you would now ask the question why on earth they bothered with a bridge deal instead of locking him up early, which would have allowed for a lower caphit...
Signing Norris was a perfectly normal decision that plenty of teams would have taken as well. Everyone and their mother tries to identify core-players and lock them into long-term deals as early as possible, because that is what you need to do if you want to be a contender. You wait too long, you have to throw tons of money at players. Not everyone would have done it, but that doesn't make it wrong or unusual at that time, because many teams would have done the same.
Look at Edmonton and Nurse. They bridged him for two years, and then he had a season in which he was 7th in Norris-voting, while having the second most goals and second most even-strength points among defensemen in the league. All while being one year away from UFA-status, with Klefbom's injury making Nurse by far the best player on the team's defense, and with other defensemen like Hamilton, Werensky or Jones signing for 9-9.5m. Without a bridge, you are looking at something like 7m at the most for a longterm deal, instead he got 9.25m due to his excellent performance that season, other defensemen getting similar deals, and Edmonton being all but forced to keep him at all cost with Klefbom's career in question. He held all the cards, something that people who trash that contract love to ignore.
This is what it means to try to succeed: you take risks to gain an advantage. Sometimes that works out, sometimes it fails, and no matter what you do, someone will always complain about why you didn't do the opposite. There is always someone who afterwards claims he knew it all along. The very same type of person rarely talks about the occasions where his claims couldn't have been more wrong though.
This is Norris' first season after a long-term injury that took him out for almost a whole year, that's not something you just ignore and go back to how you used to be right away. There is a chance that Norris will never get back on track, yes, and if he does, the bet will not have paid off, but long-term deals are signed for the long term, not to panic if one season goes wrong. He's still young, and his contract and stats are not so bad that no team would take a flyer on him to see if they could get him back on track.
The contract exists, it's not going away. The best way to move forward is to wait for what the new coach will do, and what having a healthy offseason might do for Norris' performance. After all, this season is already in the dumbster, and hardly anyone is performing well, Norris isn't going to buck the trend under those circumstances when no one else does either.
If he still isn't quite there after the circumstances have changed, then you think about alternative options.