Casemiro with his current level of play and fitness is indeed plausible that he is able to continue his contribution at a high level for three more years. He is certainly the most important player for Manchester United, even though we dont see his best performance like he used to perform at Madrid, his impact to the team is so significant. His ability to gather and control the midfield not only showcases his individual skills but also show his importance in the overall team dynamics.
Manchester United being reluctance to sell Casemiro is reasonable with the sentiment that players of his caliber. He is invaluable asset who consistently delivers and fulfills his role effectively. Manchester United as a club, especially during a recovery or rebuilding phase, would likely prioritize retaining such key players who provide stability and leadership. His experience, work ethic, and on-field contributions can serve as a stabilizing force during transitional phases.
Despite the fact that Casemiro is getting older and not very consistent with his good work rate like he used to be I think he's still got it in him, he proved it last season when he was bought from Realmadrid into united, the problem with Casemiro is fatigue, he was over used last season and it's really affecting his performance this season, in both club and country side. Well I won't blame Manchester United if they want to sell him in the January window cause they'll need reinforcement and can't wait for Casemiro's recovery.
Well if they choose to retain him based on the role he plays in the team, then it would be very understanding cause an inform Casemiro has a lot of experience and he's highly effective, which is something united could really rely on, in the long run. Well only time would tell if united would either choose to sell or retain Casemiro based on his experience and work ethics.