Why should that change TSMC decision making even a little?

The reality is that TSMC has no competition capable of shipping an equivalent product. If AI fizzles out completely, the only way Apple can choose to not use TSMC is if they decide to ship an inferior product.

A world where TSMC drains all the venture capital out of all the AI startups, using NVidia as an intermediary, and then all the bubble pops and they all go under is a perfectly happy place for TSMC. In these market conditions they are asking cash upfront. The worst that can happen is that they overbuild capacity using other people's money that they don't have to pay back, leaving them in an even more dominant position in the crash that follows.

Because apple can play hard(er) ball in 12 or 18 or 24 months when this (likely) irrational spend spree dies?

Business is a little more nuanced than this audience thinks, and it’s silly to think Apple has no leverage.