There is a reason the Celtics have won so many championships, they know how to attract and keep the best players within their concept of team and they know how to best use those players. And in my opinion, whatever helps the Celtics organization can only help the NBA. I don't see a down side to Durant signing with the Celtics.
Kevin Durant is a highly-coveted professional basketball player for the NBA. Durant has enjoyed several years of success playing for the Oklahoma City Thunder. Therefore, it is no surprise that he is being pursued by many teams. Durant should sign with the Celtics if the team offers him more money then any other team. There is nothing wrong with a professional athlete maximizing the amount of money he can earn from a team.
Yes, Kevin Durant should have signed with the Celtics. The Celtics had several things to offer, including the team infrastructure put into place by Brad Stephens, the history making potential of the resulting team and the fact that the team is in the Eastern Conference. With the Celtics, he would have made it to the Eastern Conference finals every year for the next 5 years.
Kevin Durant is a professional, independent basketball player. He is entitled to his right to sign with whomever he chooses, and the Celtics should not be the one he signs with. While the Celtics have many merits, they hold no candle to Golden State. If he is hoping to win a championship, and thus garner more acclaim and renown, he should not sign with the Celtics.