Niche product. iPad Pro 12 inch is already in a small niche by itself. Apple can make whatever size iPad they want. It is not problem of how but why. Larger iPad will require a desk and a stand so customers will likely prefer a MacBook directly, if they are not professional designers, architects or alike that count for probably less than 1/1000 of the user base. It is like the Mac Pro or iMac Pro. I still havent met a person that owns any of these devices. I dont mean Apple should abandon professional users and focus only on devices for the average customers, but at the end of the day it is what generates profits. Apple is doing anything they can to expand their portfolio of products in order to rely less on the iPhone, but I dont think they can achieve much by simply releasing random products that only exists to gain some market share and diversify the portfolio.