They could always turn to TSMC, and in that way return to the leading edge. Not as a wafer manufacturer, but as a CPU supplier, with packaging and some tiles done in-house. Isn't that the way the wind is blowing, with the talk of Intel doing some stuff on TSMC 3nm?
Also, it is no natural law that Apple is a node ahead of everybody else at TSMC. That is only a question of money. If some in the Windows world (Microsoft, HP, Dell,..) decides they must up the CPU budget, then both AMD and Intel are perfectly capable of creating large dies (as large as Apple) on the bleeding edge. AMD certainly is, if only the customers are willing to pay.