My working theory is that we'll be seeing Thunderbolt 3/4 in iPads and iPhones next year.
Is PCIe even in the Apple A-series? Since TB is really low-level, wouldn't that introduce all sorts of attack surfaces and/or insane peripheral certifications along with a power-hungry interface?
Imo, I don't see the use case. Perhaps future A-series-based Macs, I suppose.
Even if TB3 is free, it's still hard to implement. Wires need to be as noise-free as possible--that means no changing PCB layers, short but equal trace lengths with no hard corners, etc. Drivers have to be well-trusted and run as low as possible.
The point is this isn't like USB where you just add a chip and port and be done with it. TB is so cutting-edge that you have to consider physics, power, security, etc. for it to be worthwhile. And that's not including the extensive market research to find the four people on earth who would actually use TB3 instead of USB-C on such a device.