Certainly an ACL tear or other catastrophic event would have altered his trajectory.
On a human level, it is worth appreciating such things. Finding new ways to express and experience gratitude is one of the most profoundly powerful things a person can do. A true key to happiness. (I hope this doesn't sound sarcastic: I really believe this)
I think we just generally leave that sort of thing out of "what makes Lebron, Lebron?" type discussions because it's a constant for all athletes. A catastrophic injury would disrupt any player's trajectory. That's not interesting or useful to discuss from an analytic standpoint.
What we're really looking for is, "why is Lebron different from others who've avoided catastrophic events?" or to be more on-topic, "why was Levandowski able to rise to superstardom when other engineers weren't?"
Certainly, yes, some of this can be chalked up to Levandowski's avoidance of catastrophic events. He wasn't eaten by bears! His parents didn't blind him with acid! He wasn't struck by meteors! But, this is also true for a lot of other engineers, so this doesn't tell us anything useful. Everybody already knows that being killed by a bear is detrimental to one's career prospects.
To return to (and hyper-focus on) Lebron for a second...
there are a number of equal/better players (physically and mentally)
who get bumped, land wrong, tear an ACL, and their trajectory changes forever
In his case, the numbers suggest that the number of "better-than-Lebron players who don't make it" is
extremely small and likely zero.
1. Every year, millions of people play basketball and do not suffer catastrophic incidents and yet also do not display the sort of generational talents displayed by Lebron.
2. Given the extremely large sums of fame and money involved in collegiate/professional basketball, anybody manifesting his level of talent is unlikely to go undiscovered. There is a lot of incentive for everybody involved to identify and develop such talent. This would be less true for many other pursuits. I suspect there are many "undiscovered Bobby Fischers and Gary Kasparovs" out there in the chess world and many "undiscovered Lewis Hamiltons" in the racing world; I suspect this is not the case for "undiscovered Lebrons."
3. Basketball is also different from other pursuits in that freakish physical height is a great advantage. If you are tall, in most of the world, people will tell you to play basketball. A height of 6'8.5" (Lebron's height) is something like 99.99th percentile and playing basketball is one of the few lucrative things one can do with that height, unlike a 99.99th percentile intelligence or even 99.99th percentile strength.