The difference isn’t just upgrading to a 3.2 controller, it’s literally a different chip. The A16 vs the A17. Unless you’re suggesting they add a secondary controller to the board? Which doesn’t actually work, since the A16 wouldn’t be able to support the speed difference, so you’d have a 3.2 controller locked down to 2.0 specs anyway.

You ever see a comment that is so clearly ignorant. It just makes you shake your head.... congrats my friend

Crack open an iPhone sometime. The mainboard is a tiny little thing with only a couple of chips on it. In general- CPU, storage, RAM, baseband (cellular radio). Sure they could add a USB 3.2 controller, but that's another chip sucking power and taking board space, increasing BoM cost, and since most iPhone users never plug their phone into a computer it'd be wasted.

So they use the USB controller built into the SoC (system on chip), and with the old chip that's 2.0 only.
Guess they must have a surplus of A16 chips and/or the A17 is proving expensive to make.

The controller is a part of the SoC. It would be a completely different SoC, not just an additional controller. The SoC in the 15 is essentially the 14 Pro SoC. Possibly binned from last year’s production line.

Not just that, lightning was a similar speed to usb2. It's in their interest to make the pro look like an upgrade rather than highlight just how bad the lightning was really for the consumer