Yield is generally not an issue over time, at least not as big as someone outside of the industry would think, if you get enough chips to work in the first place. For high volume chips, fabs will tune their process specifically for your chip over time. And you can do mask changes just to address yield problems and you can add redundancies if you have to. For example Huawei is no longer bottlenecked by the quantities of chips they can get for their handsets. Their problem is that they can't get the performance they could get with a better process.