Qualifiers make sense. Nothing is ever guaranteed until it’s a done deal. “Tatum will get a max contract unless...” If you could think of any possible way to finish that sentence, then the max contract is likely but not guaranteed.
I honestly don't think there is an unless. Even a major injury I don't think stops him from getting a max. It would have to be basically a career ender and I just don't think that is worth a qualifier.
I think a ruptures Achilles, ACL or MCL that causes hi to miss most or all of next season would probably be enough that he doesn't get a max deal. Keep in mind the cap will now likely be falling (or at least stalled) due to no fans in the arena which will mean that few teams will have the money to make him a max offer. Combined with the uncertainty of an injury and I think he'd have to take a "prove it" deal. But of course injury aside ya, he'll get the max.
Kevin Durant says 'hi'.
Tatum is not on Durant's level lol.
He also is 22 unlike Durant who is on the wrong side of 30.
And he hasn't shown the potential to be an MVP level player like Durant at 21/22. Obviously maxes aren't restricted to MVP level players, but an All-Starry wing getting a serious injury would dampen his value quite a lot even if he's young.
I don't know, obviously numbers aren't everything but his Year 3 numbers aren't that far off, other than the scoring...his rebounding, assists, eFG, are all in the ballpark. I added Melo in for giggles.

Also keep in mind that KD was basically The Guy in OKC his 3rd year, the only other players who averaged double figures were Jeff Green and Westbrook who both averaged 15ppg. KD had to carry them for the most part and he had a higher usage % than JT did:

Compare this with Tatum who has had to share the billing with Kemba, JT, Gordon and even Smart:

Obviously he's got a way to go to develop the consistency that KD displayed his first 3 years in the league and he's nowhere near the MVP conversation but he looked to have had his breakout in Feb until the pandemic curtailed things. Put it this way if he ends up getting to Durant's level in the next couple of years I wouldn't be surprised. But yeah if he gets a serious injury it could significantly affect his earning capacity.