The problem is more the "to shun our rivals" bit.
Apple in particular always tries to have multiple sources for any parts they don't develop themselves. And even for the stuff they develop, they try to have multiple manufacturers. This is a very good idea to do at their scale. Them legally preventing from doing this by forbidding it in a contract is way different than "preventing" them from doing this by just offering them chips at super-cheap prices and hoping for them to take up your offering and to ignore all the other competing offers.
Most likely they didn't yet plan to source the LTE chips from a second supplier at the time they signed the contract, so that limit didn't affect them. Nowadays they do have multiple sources, which is probably why they did not prolong the contract any further after 2016 - because having multiple sources was worth more to them than having this discount.
What does FRAND mean?
Rights holders generally have to agree to license their patents on FRAND terms to get their technology included in wireless standards, in this case CDMA and LTE.
Qualcomm’s license pricing is, unusually in the industry, a percentage of the final retail price of the final device.
Apple’s argument is that this is not, in fact, a reasonable and non-discriminatory price for a license to use Qualcomm’s patents.
It can be. What if Qualcomm gives discount only if Apple buy as many as it need which is fairly predictable. Then Apple would have enough supply and wont need to look for another supplier.
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.
If supplier only needed few % sales from Apple, then Apple could have just given them money for other reasons (funding etc). Thus no loss of choice for consumers. This case is simply money grab.
If those % of sales are not enough, then supplier dies. Loss of choice but now due to legal means.
Apple doesn’t want to buy Qualcomm chips at any price, if it can get replacements from anywhere else. It wants reliable partners, and if possible, to contract manufacture its own baseband chip designs, without paying more to Qualcomm in ostensibly FRAND fees for chips it designs itself than just buying the chips from Qualcomm.
You can offer a -50% discount to customers (as long as you aren't selling below cost, which would also be illegal in the EU), and then you'd have to assume that the customer will stick with you as long as you offer that discount, or develop a good relationship, and so on.
But you can't enforce that the customer can't also buy the product at 100% of the price or whatever it is from a competitor at the same time it's buying it at -50% from you. If the customer does that, it should be their choice. You can't ban the transactions with your competitors in the deal.