I would say that it really depends on how you think about "the system."

If the hardware is in-scope for the system, you're generally out of luck at having provable guarantees unless you're willing to operate a foundry and courier your own products.

I think it comes down to threat modeling. You have some threshold where it's unreasonable for someone to attack a system for less than $X spend. Right now on iOS I think that's on the order of $500k-$1m usd.