I get your point. If you are 100% wedded to Intel, then even if Intel never release another product, a 14nm, PCIe 3.0 CPU will stay state of the art until the end of time.
But if Apple are capable of ditching Intel for ARM, they are certainly capable of ditching them for AMD. I expect, though, that given the many advantages for Apple of doing the former, they weren't about to jump ship to AMD for the final year or two of x86.
Intel may also have been giving Apple steep discounts on their CPUs, to stop them jumping to AMD. On a similar note, I expect AMD are more flexible on pricing than Nvidia are, which is why Apple uses their GPUs exclusively. As the sole hardware supplier to the macOS platform, Apple get to chose what gives the best margins.
Not necessarily, it could be a contractual agreement that specifically mentions AMD. Or its a contractual agreement that they did not want to break just for AMD for a year then move to their own processors. We just do not know the behind the scenes legalities of this. Businesses do not have that kind of flexibility that people think they do.