I agree, but I think it's much more likely that Apple simply did not consider this case, rather than actively working against having commonly-available replacem

I agree, but I think it's much more likely that Apple simply did not consider this case, rather than actively working against having commonly-available replacement components on every board. Apple does not replace components, they replace motherboards. I think you'll find that as volume increases, this kind of practice becomes more common, as a way to reduce cost, increase yield, reduce board size, etc. And, that this will be true for even lower volumes in the future as customized silicon becomes more feasible at lower volumes, such as 10k or 100k units, as we're seeing with companies like SiFive popping up.