Consider what a downward sloping demand curve means. The first customer is willing to pay a higher price than the second customer and so on.
Concretely, consider a first customer who is willing to pay \$1 million and a second customer willing to pay \$500 thousand. So if Nike prices at \$1 million a pair, it makes one pair and sells for \$1 million. If it prices at \$500 thousand, it makes two pairs and sells for \$1 million total. This is not discriminatory pricing. With discriminatory pricing, Nike would sell the first pair at \$1 million and the second pair at \$500 thousand, for \$1.5 million total.
Now consider what happens if we add a third customer who is willing to pay \$100 thousand. If Nike sells at that price, it has three customers and sells for \$300 thousand total. It was much better off selling at either \$1 million or \$500 thousand. But what if there are more customers at \$100 thousand? If we add ninety-seven more customers at \$100 thousand, Nike can sell a hundred pairs for \$10 million total. That's more than it made at a \$1 million price point.
Your assumption is that if Nike sells its shoes for \$300 and there is someone willing to pay \$2000 for a pair, that Nike could sell at \$2000. But if Nike sold at \$2000, many of its other customers would drop out. That's not a market price; it's a specific customer price. And if Nike sells less than a seventh as many shoes at \$2000 as at \$300, it makes less money even with the higher price.
To explain why they price at a point where they run out rather than leaving a slight excess. Because forcing people to make a \$300 or no shoe decision may be more successful if the person knows that the shoe won't be available tomorrow. They may actually sell more shoes if they are expected to run out than they would if the shoes were guaranteed to be plentiful. Because people can't procrastinate making the choice.
That's not to say that there aren't marketing reasons as well, but that we don't necessarily need marketing to explain this.