Scoring 50 goals a year for twenty years does not necessarily make you the best goal-scorer of all time. It makes you very consistent and long-lasting. Do an era-adjusted ranking, and there's a shmozzle of players with peaks in the same neighborhood, including Hull, Lemieux, Esposito, and Gretzky. Ovechkin will always be one of the best in that framework, but his consistency and longevity do not necessarily imply he had the greatest peak.
So holding the all-time record would matter a great deal to his legacy. It's one thing that he can hold that can't be disputed or qualified.
Gretzky's claim to the title is still meaningful even if Ovechkin passes him, though. Because Gretzky hit such an absurd number while goalscoring was an ancillary aspect of his offensive game. If he'd tried to play a more one-dimensional game, he might have hit 1000, while being a much less effective player overall. So there's always going to be an argument about raw talent and impact on the game at your peak.