It's not the problem that Sabledonk is going to win Nats, it's the problem that Sabledonk prevents good players from winning Nats cause they lose to luck. Sure, this already can happen, expecially in a format like the one we have, but Sabledonk makes things a lot worse.
You have a basic matchup at the beginning of the game, this is determined by the decks and by the techs players have in their deck. But the longer a game is played, the more the odds of winning shift towards the more experienced players, because he makes less mistakes and can plan ahead better. That's why better players win more tournaments. If it's a Luxchomp mirror with same lists, the basic matchups is 50-50, but the longer the game goes, the higher the chance that the better player wins. And this is why a basic matchup of 50-50 or higher is enough for a good player to get consistant wins.
Now with Sabledonk, the game goes for one turn. Either you donk T1, or you scoop. Sure, Uxie donk can keep playing a little more with infinite Psychic Restore and PlusPowers, then Setup then back to the hand again. Sabledonk will have problem to do so, since the one energy is attached to Sableye and there's no chance to get it back. So if the basic matchup is let's say 75% against normal decks and 50% against Spiritomb decks, it's the final matchup that can't be altered by either players skill (unless the Sabledonk players' skill is too low to play the deck correctly).
That means, if you run Sabledonk and assume that at least half of the opponents in your tournament run either 4 Spiritomb or 4 Sableye, you'll win about 62% of your matches statistically. That's not enough for making the cut, so you'll need a portion of luck that's above average to make it.
So why is Sabledonk still a threat for the format? Well, image a random supporter card that says "Flip a coin. If heads, you win. If tails, you lose." Sabledonk is basically this. The player can choose to determine the match outcome by luck, and there's nothing an opponent can do against it. And this is not the point of a game. It's not fun, it's not fair, you won't learn anything from it but how it feels to be frustrated... well and that's already half of the points of the Spirit of the Game that are hurt by Sabledonk.