Apple was just the highest bidder for getting the latest TSMC process. They wouldn't have had a problem getting other customers to buy up that capacity. And Intel's missteps counted for a substantial part of the process dominance you refer to. So I'd argue that Apple isn't that special here either.
Until Apple forced other chip makers to respond, nobody else was making high end phone processors. And their A series processors are competitive with and have transistor counts comparable to most mobile and desktop PC processors (and have for years). So the alternate reality where Apple isn't a TSMC customer means that TSMC is no longer manufacturing several hundred million high transistor count processors per year. In my opinion, it’s pretty likely TSMC isn’t able to achieve or maintain process dominance without that.
Update: To give an idea of the scales involved here, Apple had iPhone revenue in 2024 of about $200B. At an average selling price of $1k, we get 200 million units. Thats a ballpark estimate, they don’t release unit volumes, AFAIK. This link from IDC[1] has the global PC market in 2024 at about 267 million units. Apple also has iPads and Macs, so their unit processor volume is roughly comparable to the entire PC market. But, and this is hugely important: every single processor that Apple ships is comparable in performance (and, thus, transistor counts) to high end PC processors. So their transistor volume probably exceeds the entire PC CPU market. And the majority of it is fabbed on TSMC’s leading process node in any given year.
[1]: https://my.idc.com/getdoc.jsp?containerId=prUS53061925
Exactly. This is why competition is good. Intel really didn't have a reason to push as hard.