Actually everyone has a legacy. The term doesn't only refer to greatnesss. There can be a legacy of greatness, above average play, underachieving, mediocrity, disappointment ...
If talking about greatness, I agree that Dak doesn't have that legacy.
But how many QB's do without playoff success?
Aikman wouldn't have that legacy without the playoff success. His stats weren't good enough on their own.
With Dak, his legacy is still to be determined, but for now it's obviously not what Cowboy fans would hope for.