The "single monopoly profit theory" is often viewed as quite counter-intuitive:

Leverage of market power cannot be used to profitably foreclose a rival.

Suppose there are two products, A and B. A is monopolised and produced only by firm 1; B is supplied competitively by both firm 1 and firm 2. Common sense presents to following concern: firm 1 might try to use its market power in A to become a monopolist in B and foreclose competition from firm 2. One way to do this would be to bundle A and B1 together. Everyone who buys A would also be forced to buy B1, even if B2 were the better product. This would make it difficult or impossible for 2 to achieve any sales.

This logic is flawed, as the Chicago school pointed out. Suppose consumers will pay $v$ for A, $v$ for B1, or $v+\Delta$ for B2 (so $\Delta$ is firm 2's quality advantage). Suppose that A and B1 are bundled and that consumers buy the bundle. In a desparate attempt to win business, 2 will cut the price of B2 to zero (this is the usual Bertrand logic). Thus, consumers will buy the bundle if $$2v-p_1\geq v+\Delta\implies p_1\leq v-\Delta.$$

Thus, the best 1 can do through bundling is to earn $v-\Delta$ from each sale of A and B1. But it could do better simply by selling A at a price of $v$ and giving $B_1$ away for free. Thus, the claim that bundling/leverage of market power is a profitable way to foreclose competition turns out to be logically flawed.

Addendum: subsequent work has shown that leverage of market power is possible in various situations. But the conditions needed for it to work are more intricate than suggested by common intuition.