If Horford opts out he isn't signing a 3 year deal at $16 million per. That is the equivalent to him staying on his current contract and playing the next two years at money less than the MLE. Say what you want about his age, but Horford's skill set is still one of the most sought after in the league and he has aged very well. If he didn't want to renegotiate and just opts in, he would command much more than MLE money after next year.
Orlando isn't trading a fully healthy, young and still room to grow Gordon for a possibly still struggling to be 100% fully healthy, both physically and mentally, Hayward. If their GM did that the Magic owner should instantly fire him.