This is what I was referring to, and we already have seen this happen in the wild with PayPal at one point (possibly still) blocking passkeys from e.g. Firefox. For now the argument against this seems to be that "Apple zeroes this out so service providers can't do it without risking issues for their many users who use Apple to store their keys", but clearly this is so precarious of a situation it may as well not be a thing. You can't depend on one trillion-dollar company not changing their minds on that tomorrow.

Even with the current flimsy "What about iPhones?" defense against attestation, is there anything stopping say Microsoft from just forcing you to install a different app to use Microsoft services?