Hogan worked because of many different things, the physique, the name, the way he was pushed and the exposure Rocky got him. Make no mistake, the image of him with Stallone in a Gorilla Press, hurling him out of the ring did more to make Hogan than anything Verne Gagne ever did and by the time Vince got him, the momentum was there.
Flair wouldn't have caught on in the same way... he was a natural heel, worked best when he was a cowardly cheat...many of his best spots were the begging off etc...
For Flair to have been the poster child of the WWF would have meant Vince having to focus on "negatives", like the plane crash he survived. He'd have been the opposite of the "strong" wrestler, however inspirational his tale of recover was... just as if Magnum TA had come back after that crash... he'd have been seen as a plucky survivor rather than the superhuman hero.
The other thing that was essential was the name... Hulk... it had resonance as the TV show had only just ended and was for 5 years in the top 10.... EVERYONE knew the Hulk and the wrestler who was named after him was always going to be the big star.
Could ANYONE have replaced him... Slaughter could have potentially...Barry Windham could have if he were a year or two older... Kerry Von Erich could have if all the Von Erichs had signed... but that's about it.