Would they? Apple already has the best chips. What makes you think that the average consumer cares that much about even a 20-30% performance difference. It is not like TSMC is the only competent chip manufacturer. Nvidia largely uses Samsung and they are doing just fine.
Apple already has the best chips, but in this case no one else would have them. Samsung is not doing fine, Nvidia switched off of them after a single generation because their process isn't very good.
They become part of apple. With a stranglehold on chips the price of apple products as a whole, or their sales, will skyrocket.