Would it be feasible for them to buy Intel instead? Starting your own foundry would likely take over a decade.

Yup; or potentially just purchasing a fab from them, given that Intel has signaled they want to leverage TSMC more, and much of Intel's remaining value is wrapped up in server-grade chips that Apple wouldn't be interested in.

But also; Apple is one of the very few companies at their size that seems to have the political environment to make, and more importantly succeed, at decade investments. The iPhone wasn't an obvious success for 5 or 6 years. They started designing their own iPhone chips ~the iPhone 4 iirc, and pundits remarked: this isn't a good idea; today, the M5 in the iPad Pro outperforms every chip made by EVERYONE else in the world, by 25%, at a tenth the power draw and no active cooling (e.g. 9950X3D). Apple Maps (enough said). We're seeing similar investments today, things we could call "failures" that in 10 years we'll think were obviously going to be successful (cough vision pro).