If he isn't cleared for a buyout this offseason, I would not buy him out in 2025-26 because he no longer would qualify for the 1/3rd ratio. The only way a buyout works is if he can be cleared for a buyout by the second buyout window. The exact timeline for his recovery has not been disclosed other than informing fans that he expects to start the season healthy. (I don't remember if Norris starting the season was an official thing, a rumor through Garrioch, or is an urban legend.)
I don't think he was a five million dollar player this year. Consistency is important. They also need to factor in a few other things, like how his performance projects going forward after this injury. Or whether they expect him to be in and out of the lineup. Another factor might be insurance. We aren't a budget team, but every team still has some sort of ceiling of what they can spend, except for a few like Toronto and the Rangers. If his shoulder injury was a pre-existing condition, he may not be covered by insurance if he goes to Clarke MacArthur Cove (LTIRetired). A buyout (if medically cleared) could save the team tens of millions of dollars in a scenario where they project that another shoulder injury could end the career of Norris.
You're right that you do need to factor in the cost a true replacement player, but you also need to factor in liability. Even if it would cost a few million to replace the production or impact of Norris, the player who replaces Norris is unlikely to get significant term. Norris is signed for 6 more years at 8 million. An inconsistent 50 point center is a 4.5M x 3 type contract unless they bring significant intangibles elsewhere. Even if you disagree with the exact assessment of the cost to replace him, the point is that whoever they replace him with will not only be cheaper (even with the buyout) it will come with less commitment and liability.
Lastly, you have to factor in the level of importance this season has both to the players who have been here losing for a decade and to the fans who shelled out money for tickets this year in good faith. The "I'm not Melnyk" honeymoon phase will only last for so long. If the idea is that Norris will be ready to start the season, then he has set backs, or has to be shut down for points throughout the season, or worse is simply ineffective due to missing so much hockey and dealing with his shoulder, that is going to severely handicap the ability for the team to compete. It's like taking almost 10 percent of the cap and lighting it on fire. The team is already low on cap flexibility with multiple additions to make. Unless his medical records indicate that he will be great next year, the riskier play is keeping him and losing the ability to spend that 8M on other additions that could help the team finally make the playoffs.
