Letting KG and Doc (and possibly Pierce) compete for a championship instead of suffering though this season is a lack of loyalty? If without the trading Doc decided to take a year off and KG decided to retire, how is that a more noble scenario?
Anyway, the only loyalty Danny should have is to the team. If that means he needs to move a player, he better. Pierce signed a deal with a team option. Why wouldn't the team decide to not pick it up if it made sense? That is the contract he agreed to with the understanding that he would get paid well in the guaranteed seasons.
If you don't like that trade, that is your prerogative. But the 'loyalty' talk is not compelling.