Leslie's Law is about the cycle in technology: small, simple and cheap supplanting large, complex and expensive solutions.
When a sleek, small player enters the market, it does so by creating a low-friction, high-fit and finish product that is sold at a low price to a large market. These new products are sold to a portion of the market that cannot access the larger products due to the cost of entry (in dollars and complexity) and the cost of ownership. The larger company may not even notice that they have entered the market since there are no mano-A-mano customer confrontations.
This leaves the smaller company free to expand upward into the market. Their leading edge customers whose needs are expanding, and their own interest in expanding their market upward spurs them on to increase the features and functionality of their products. From the perspective of the large incumbent companies this upward migration is imperceptible. They aren't worried so they don't pay attention to it. But it’s happening.