General business intelligence: You do NOT want one of your 'verticals' (companies that you need to buy supplies from, or companies that you sell your product to) to turn monopolistic; after all, once they are a monopoly they can really squeeze you out.
This holds even if you are also nearing a monopoly or are an effective monopoly (in that you're the only one that can supply it at the quantity and quality required).
There are mitigating circumstances, but most of them don't look good for apple:
* TSMC is stupid. Don't knock it - I've seen companies go for the quick buck, then get killed by the monopolistic monolith they enabled and be surprised.
* TSMC laughs in the face of this and simply isn't taking an apple near monopoly on chips particularly seriously. I can definitely see that - the risk is presumably that apple gains massive increases in marketshare of PCs/laptops and smartphones, but TSMC presumably doesn't think it'll be so high as to risk the situation that TSMC can produce more chips than non-apple buyers could buy. In other words, that apple's total 'TSMC fab capacity' needs won't go anywhere near 50% of what TSMC is likely to ever be able to manufacture.
* Apple is putting pressure on TSMC. TSMC knows this is a risk and doesn't want to, but apple does want this to happen and is making some shady deals so that TSMC does the math and decided that the gains in dealing with apple exceed the losses*odds-it-will-happen of the squeeze if apple is a monopoly buyer for TSMC (as in, of dubious legality and certainly of questionable morality, but then, it's apple, a company. Waiting for companies to act morally is silly, you write laws and set up societal systems to incentivize them to do so instead, companies are amoral (not immoral - they just don't really know what it is, by design).
Your comment seems to suggest it's a smart move for TSMC to just sign one giant deal with apple and be done with it: You can send the lawyers and salesfolk to early retirement, get some money upfront, guarantee 100% sales of all your fab capacity with a party that is unlikely to renege or declare bankruptcy.
It's not. It's a dumb move. Either the C-level execs at TSMC are missing something pretty fundamental or more likely we don't know enough to realize that there's sufficient weights on the other side of the balance to counteract the downside of enabling the monopoly on verticals.