The real reason Apple would choose to do this is for economies of scale. There are lots of new devices with the A12, and none with the A10X, besides the Apple TV 4K.
The A12 was used in a lot of devices (economies of scale)
I think you guys are half misunderstanding the economies of scale with regard to semiconductor fabrication. There are economies of scale, but the real savings are about lifetime total output rather than rate of production. In short, it's not about how many of each chip Apple is having made per day today, but rather how many of each chip Apple has had made total since the beginning.
For example, with some simple things like injection molded toys, the rate of production is what drives the savings. In other words, the more you make per day, the cheaper each one is. But with semiconductors, this is true, but it's not as big of a factor. This is because with simple things, the upfront cost is low relative to the material and ongoing cost of keeping the lines going.
With semiconductor fabbing, the front-end costs are MASSIVE. Initial R&D, then setup/layout, then many super expensive test runs; basically a very high % of the lifetime cost of the project is paid before you manufacture a single production chip. But the materials and ongoing costs are low relative to everything else. Because of this, each chip manufactured makes them all cost a bit less. So even down the road, when you're not making that many per day anymore, it's still worthwhile to keep making older design chips at a slow rate. By that point, the cost per chip is so low, so why not?
So it's not THAT important that there are no other A10X devices being sold today. The important thing is that over the lifetime of the A10X, a lot of devices have been sold.
Also, I don't necessarily agree that Apple is only making A10X chips for the ATV today. Don't forget to account for refurbs and repair parts - many of those A10X-based iPad Pros are still under warranty and will be for a while, and some countries require Apple to keep spare parts in stock for a number of years.
That said, I agree that switching to the A12 is probably cheaper. In total sum, there have probably been more A12 devices sold by now than A10X devices (maybe even A12X/Z too). Also, the A10X is an early 10nm FinFet design which is complicated - my understanding is that TSMC can fab the newer 7nm designs easier today, which is what the A12 is. It would also allow Apple to basically move entirely to the 7nm or smaller processes, no longer needing to purchase space on the 10nm/16nm production lines.
