It basically comes down to this:
Are you more loyal to Pierce and KG or the Celtics? As a Pierce and KG fan, I'd like to see them both retire Celtics and ride off into the sun, but, as a Celtics fan, I'd rather get something more than simply cap space in return.
Plus, realistically, coaches don't make great teams, players do. Doc was a decent but not great coach before the "Big 3".