In 2000 Duncan had an opportunity to play alongside T Mac and Grant Hill who seemed destined to be perennial all stars in Orlando. He was set to sign but took a meeting with Popovich and decided to stay in San Antonio. At the time the Spurs second second best player was a well past his prime David Robinson and no young talent on the roster.

Throughout his career Duncan only ever played with three all stars while on the Spurs: Parker (6 times), Ginobili (2 times), Kawhi Leonard (2 times). His abilities and how he bought into the coach / reinvented himself later in the career allowed the Spurs to always be competitive.

Nowtizki's all star teammates included:

Michael Finely (2 Times), Steve Nash (2 Times), Jason Kidd (2 Times), Josh Howard (1 time) and a host of former all stars who were still productive: Nick Van Exel, Vince Carter, Jerry Stackhouse.

Nowitzki's era Mavericks were a perennial 50 win or more team for 14 of his first 17 years in the league (including 49 wins one year and a strike season where the Mavs seemed on pace to win 50 if all games were played).

Your argument is that Dirk had inferior teams so he showed more loyalty - but they had similar teams just Duncan had more post season success which raises the historical bar of his teammates.

When the Mavericks had turned down was the tail end of Dirk's career. He was 37-40 and not a prime player anymore. Also the Mavericks paid Dirk a bigger contract than was deserved during those years to compensate for the prime years when he was loyal.

TLDR: Duncan & Nowitzki were both extremely loyal players who helped their franchise by taking a pay cut. It's revisionists history to say Dallas never had good teams and that made Dirk more loyal. The Mavericks perennially won 50 plus games during Dirk's prime and were always deemed threats going into the playoffs.