Most people are just using the ISP provided router as their gateway today anyways. E.g. ATT fiber is proud to advertise to you that it knows about each of your devices on the ONT+Router combo - that's even the only way to set up a port forward (you can't just type in an IP, you have to pick a discovered device).

"But people can NAT the v4 with another router to hide it!" -> sure, and the same crappy solution works with v6.

"But at least prosumers can replace the ONT via cloning the identifiers and certain hardware" -> also no change with v6.

Randomized addresses do have valid use cases though, particularly when connecting to Wi-Fi networks other than your own when set to randomize the MAC per connection (not just the scanning MAC) as well, but I'm just not really convinced this is a realistic example as framed.

I think you just changed my mind. I hadn’t thought about that angle.

If ISPs tried that, everyone would just go back to using NAT, even for IPv6.