A GM who won't do what is best for the franchise and his bosses because of sentimentality should be fired. You don't not improve your team because a player "earned" some mythical "right". You contend for championships. That's the job. That's what you are hired to do. You don't let a player decide if he wants to be traded or not. I feel the same way with players. If you are a UFA you do what is best for yourself. If that's staying where you are comfortable and happy, cool. If that's going somewhere for more $ or a better chance to win.. that's fine too.
You can give the player a jersey retirement ceremony in the future. But if you have a chance to get a future cornerstone piece for 2 or 3 months of a player then you'd be crazy not to do it.
Brad is no better a player or legend of his team than Paul Pierce was to the C's. But Ainge made a great trade dealing Pierce and you see the fruits of it now.
Jaylen Brown and Jayson Tatum for the remaining years of Pierce and Garnett was what was best for the teams and the fans.