To expand on this great example a little: TSMC is a really smart case, because the chip business is all about extraordinarily high up-front development costs and near-zero marginal costs.

TSMC uses Apple as their first-and-best customer for their bleeding edge chips, using fat stacks of iPhone cash to push the limits of semiconductor technology.

Then they can keep selling chips built on that process for a decade or more (to a very long tail of other customers).

Nobody would say that TSMC isn't dependent on Apple! They're certainly tied very tightly. But they've turned that dependency into just the top of a very profitable funnel.

TSMC has many customers. If they lost Apple (and I don't see how) they could have to make some hard decisions but they would still be the world's top fab.