"All time" is being interpreted depending on how people interpret "greatest". If greatest means "highest skill ever displayed" then you're still looking at the entire history of melee (all time), but longevity isn't needed. Traditionally speaking though, it's generally accepted that the term "GOAT" does imply longevity to the success.That's really shaky logic considering that the phrase "Greatest of all time" includes *all time*. In other words, longevity is needed to truly attain that title.
You have to ask yourself... If Ken never stopped losing, and won every tourney from 2006-2016, and a random guy entered the largest tourney ever and 4-stocked every relevant player on earth, including Ken, and then disappeared, is Ken the goat, or the man that clearly dominated the world one time (exhibiting a skill level clearly above Ken's ceiling)?