The discussion about whether this could overtake the World Championships in popularity or prestige seemed kind of odd. Grant seems really bought into this format, but that seems a bit overenthusiastic. The benefit of the world championship name is that it is legible to marginal fans. Even if you don't know anything about the sport you'll understand that being a world champion is a big deal. Grand Slam is more opaque and who knows if it will even be around in a couple years.
The other thing he's missing is that at the World Champs people rep their countries, but at Grand Slam people are just repping their sponsors. That is a huge because patriotic sentiment will cause people to cheer for athletes even if they didn't already know them. This also results in the absence of relays which are some of the most popular events and great for national rivalries.
Third point is that at the World Championships it builds to a final race where everything is on the line. This is a proven format in most sports because it builds tension and desperation.