The way that could play out is that Nvidia wouldn't buy directly from them, but somebody else purchases cheaper from TSMC and then Nvidia buys from them.
So TSMC would have to increase prices for everyone, but then for some it would be too expensive and so they would lose those customers.
Nvidia engineers work shoulder to shoulder with TSMC to squeeze everything from the architecture even before the fab is build. TSMC makes custom job for their best customers.
For old established process it's possible to send design and get functioning chips without a huge R&D hassle and loss of performance.
So TSMC would have to increase prices for everyone, but then for some it would be too expensive and so they would lose those customers.