Are they going to be able to supply enough True Depth cameras to be able to do that? I think the priority is getting enough to move it to the entire iPhone line before jumping to iPads and Macs. Probably won't see a face ID enabled iPad until 2019 at the earliest.
Sure they can. I agree Macs likely wont be seeing Face ID anytime soon, but iPad Pros just don't sell in the numbers that would significantly impact iPhone yields, especially as their release cycle is heavily staggered from that of the phone.