I think at this point the bridge between Apple and Qualcomm is pretty well burned, especially after all the legal nastiness. I'm wondering if Apple will move some of its chip design talent to fabricating their own modems. It seems that Apple has been trying to pull more stuff in-house for greater integration and performance (plus more profit). This is also supported a job posting someone uncovered: https://bgr.com/2018/05/01/iphone-5g-chip-apple-qualcomm-int...
Apple really doesn't like putting all their eggs in a single vendor basket.
It was my understanding that when Apple started using both Intel and Qualcomm modems that Qualcomm started telling Apple they would have to pay "full price" for patent licensing on the Intel modems, and things deteriorated.
I believe cost is only one part of the reason. The much more important part is, I think, control.