The chance of that happing is close to nill.
But it's also worth noting that apple is expanding is manufacturing in other countries, notably India, Vietnam and Brazil, so they are somewhat hedged on that already.
The FT reported that 50% of Indian iPhone _casings_ did not pass quality checks [1] to be sent forward for factory use. That is just the casing, a not particularly complex piece relative to other iPhone parts. On top of that, Mac Pro production moved from the US back to China [2] after an attempt at "Made in America" due to easier parts sourcing.
I can see Vietnam getting there, but again the scale is just not comparable to China.
[1] https://www.ft.com/content/0d70a823-0fba-49ae-a453-2518afcb0...
[2] https://www.ft.com/content/0cddd49e-99d2-11e9-9573-ee5cbb98e...
Not nil as long as Taiwan is still "independent".