In that case Apple is still free to just buy a few percent of the chips they need from a different supplier, use those as well in production and in the worst case shelve the surplus chips from the Qualcomm supply agreement (if the quantities set in the agreement perfectly match the needs of Apple at that moment).

Whether this makes economic sense depends on whether savings_due_to_discount - (price_of_chips_from_second_supplier + lost_value_of_surplus_chips) works out for Apple. That may be a negative number, which means that Apple has these costs for getting the majority of chips from Qualcomm at the super-cheap discount while still having a second supplier actively used in production. But these costs may still be worth the risk mitigation that occurs when having a second supplier in the chain. Especially for Apple, for which lost revenue due to supply constraints can quickly eat up any marginal savings on one component in the supply chain.

The point is: it is an economic decision, which is exactly what it should be, and the decision can be made and re-evaluated at any time. If you exclude second suppliers by contract, it's only an economic decision when the contract is signed, after that the economic decision becomes a non-decision, even if the economic rationale changes (for example because a competitor became better). And that is exactly why this contract clause is anti-competitive and was deemed illegal.