If you compare his numbers and longevity with other Cs greats in totality I can see what your're saying. But if you compare his impact on the franchise, one could say it was comparable with other great Cs. Celtics was irrelevant for 2 decades before he arrived. He won a chip his first year, and every year thereafter the expectations were championship. Before KG, the Cs were probably 4th behind the Pats, Sox, and Bs in the area. When he joined, the Cs were again on par with the with the Pats and Sox. That sentiment towards the team has carried over to the current team where the fans expect a championship team. KG coming to Boston was the lynchpin that changed all of that.
I don't know why the Timberwolves haven't retired his number