Apple can't easily go to a competitor. They have the cash on hand to play kingmaker though. They could go to one of TSMC's competitors and make them into the new TSMC. TSMC's current position is largely due to Apple fronting huge sums of cash for them to develop new nodes. They get guaranteed sales at a generous price for the first few million chips off the line. There's few other players, if any, that could offer TSMC a deal like that.

Apple could with some pain replace TSMC but TSMC can't really replace Apple. While Apple is a demanding partner they pay their bills on time and can and do front R&D money. Once Apple's deliveries are made TSMC now has a money printer on the new process node Apple paid for.