Players around the league love Carmelo Anthony and want him back on an NBA roster. League executives, in a vacuum, think he belongs in the league. When I've talked to team sources about 'Melo, the general response is something like "he absolutely could still play in the league, he's just not a fit with us." Team executives have concerns about his willingness to accept a bench role and his midrange-heavy game.
However, it may be his defensive issues that have him on the sidelines entering this NBA season (and maybe for the rest of his career).
Baxter Holmes of ESPN did some fantastic reporting talking to Rockets and other executives about Anthony and the issues often came back to defense.
Anthony's defensive woes were most evident in a 2017 playoff series against Utah, when the Jazz dragged him into every pick-and-roll they could, and in the end the Thunder were 29.4 points per 100 possessions better when Anthony sat vs. when he played (stats via NBA.com). By the end, Anthony sat a lot.
That defense is a concern for a team that is going to give Anthony critical or heavy minutes.
However, if he's willing to accept a bench role that will likely not see him in games at the end, the defensive issues are less of a problem. If Anthony is just on the court for 15 minutes a night and is tasked with just getting buckets (pair him with better defenders), he can help a team. The fit just has to be right, and as the season starts some team or teams will realize they could use a guy like Anthony off the bench.
In a league where Dwight Howard gets one last chance to play a role, Anthony deserves a chance, too.