Both Ric Flair and Dusty Rhodes (Jim Crockett's two top stars) wanted Magnum as NWA World Champ. Crockett himself wanted Magnum as NWA World Champ. Crockett had considerable influence with the NWA Board of Governors. Flair's word (as World Champ) carried a lot of weight with the Board, as well.
Magnum was a fast-rising star throughout Crockett's large territory, very popular wherever he wrestled. High-profile feuds with the likes of Tully Blanchard and Nikita Koloff made him one of Crockett's marquee stars. As did his tag team with Dusty Rhodes in Rhodes' war with The Four Horsemen.
Magnum would have beaten Flair for the NWA World Title if he hadn't have had the car wreck. He would have been a very popular World Champ, possibly even rivaling Flair. He certainly didn't have Flair's flamboyance and showmanship (nobody did, really) but he was the "working class hero" that fans loved in those days. He likely would have been NWA World Champ a few times for several years, battling Flair and the other heels. Nikita Koloff would have remained heel and been a top challenger to Magnum, likely being World Champ himself a time or two. Sting's rise as the new Face of the NWA/WCW would have been delayed but not stopped. Sting had more universal appeal than Magnum, who was a little "too southern" to really challenge Hogan for worldwide dominance. Hmm, it would have been interesting to see how Sting as Crockett's top star would have stacked up to Hogan (in worldwide popularity) in the 80's when Hulkamania "ran wild". A question we'll never know the answer to...
Magnum would have been a very popular NWA World Champ but nothing was going to stop Vince McMahon from destroying the NWA, putting everybody out of business, and taking over the wrestling business in North America. Magnum would have continued the tradition of NWA World Champs traveling the territories (and other countries) defending the World Title and he would have been a top draw wherever he went. As Vince's death march on the wrestling business grew stronger, Crockett would have brought Magnum "home" and kept him there (as he did Flair) in a desperate attempt to hold Vince off.
But it wouldn't have made any difference. Vince was too rich, too powerful, too cold-blooded and ruthless, and was willing to do anything (and did!) to put all competition out of business.
The NWA was an alliance of territories and promotions throughout the country, they mostly cooperated and worked together for the betterment of the business. Crockett wasn't trying to drive anybody out of business, and neither were Flair, Rhodes, and Magnum. Vince WAS and did everything he could (fair and foul) to do just that.