nike was a younger company compared to adidas, reebok or puma or converse....they needed that one big signing move to strike real hard, and they did with MJ. But MJ was not the only reason why nike became so big. Their creativity is what got them to this place, they were making better designed shoes than the rest of companies. I mean shoes from the 80s like air force 1 or air max are worn up to this day, their technologies were better, the nike air technology really taking the shoes to the next level.
So we are talking about better looks and better performance. Once MJ joined Nike they had easier chances to sign other top athletes too, they made a great move by expanding their shoes to many sports, not just concentrating on a few. And then they dropped the "just do it" commercial, which made any person from audience, fat or skinny like a muthafukka, to believe in themselves and to start exercising. Those commercials was as groundbreaking as apple company commercials when they made the audience feel like they were in commericlal, by showing scenes from the everyday life situations.
Pick for example reebok, when they had iverson, AI of course was no MJ, but at one point he was the most popular player, the shoes however were ugly, and this day nobody is wearing answer's, besides vintage fans, neither shaqnosis or kamikaze or any popular reebok shoes from the 90s
So basically if nike had MJ and they didn't know how to make good looking shoes, their shoes wouldnt be selling that much even if it was MJ himself on their cover. Innovation is what made nike stand out from the rest of these companies.