Playing for a decade doesn't make someone a legend. No one has ever claimed that to be the only thing that makes you a legend. The reason someone like Taz is a legend, but someone like SIXER isn't is because of their accomplishments over that time.
If we count the number of Top 3 finishes at Premier level tournaments then Taz has finished 3rd at 1 (WESG 2016), 2nd at 2 (Epicenter 2016 and Eleague Major 2017) and then 1st at 7 events (WCG 2006, EMS One Katowice 2014 (a CS:GO Major), ESEA Season 18, EPL Invitational, Eleague S1, DH Bucharest 2016, and DH Las Vegas 2017). This doesn't account for all his top finishes at smaller but still highly regarded events in 1.6 and GO.
SIXER on the other hand has only had one 1st place finish at a premier level event in his career and that was WESG 2016.
Results are what separate the failed pros from the legends. People like TAZ, NEO, forest, get_right, etc. are legends. They have a history of winning events and doing it fairly consistently.