I'll add up on skizz's post.

There are some OGs legends who was recognized due to them being really skillfull. The perfect example would be Leyz or Ticux.
They were one of the first if not the first top notch mp players and the skill gap between them and majority of community was HUGE, they were skill gaming pioneers.
Diamond also got that cuz he was really good without competition in a while. But now we have loads of really strong players like Shev, iTemp, Kros etc.

If we will think "Toricredits earned from skill" those guys back in the days was DOMINANT, nowadays it is more like pouring huge TC bank from one cup to another, cuz all top duelers are good and their winning/losing is caused by unnoticeable things. (tho iTemp for me personally stands out)

In general getting legend status should be via being a community staple (e.g. Nirs) or being a pioneer in something.
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UPD: Join Date has to do nothing with getting a piss name