I don't think anyone has argued that it does. I've argued two things in respect to this situation:
1. In the example you gave, it should neither tarnish nor enhance someone's legacy.
2, If two players have almost identical careers but one has a bit tacked on the end or the start of playing not to the average standard produced in the rest of the career but still positively contributing, it does enhance their legacy.
When it comes to Tendulkar, his performances when he was 38-40 should neither tarnish nor enhance his legacy -- he was crap and might and might as well have not been playing -- and his performances as a teenager should enhance it -- they weren't to the standard of the meat of his career, but most players aren't selected at that age and it was a positive that he could contribute. The beginning part, unlike the very end, was a genuine positive.
TH was genuinely arguing that, like weldone thought you were saying, if two players have identical careers up to a point and then one retires but the other keeps playing and drops off a bit, or one debuted at an earlier age and wasn't quite as good, the one who played less is better. I don't rate Tendulkar higher now than I would if he retired 18 months earlier because he did nothing, but many people rate him lower because his average dropped and his record supposedly suffered, and that's what I'm making the case against. Playing on should never tarnish one's legacy but it obviously doesn't always enhance it either; you do have to actually play decently or it's the same as having retired.