That's my point, though. mikeryan argued that if Apple didn't force devs to support their auth solution, devs would just ignore it. But both Google and Facebook

That's my point, though. mikeryan argued that if Apple didn't force devs to support their auth solution, devs would just ignore it. But both Google and Facebook have created similar solutions and have been very successful without forcing devs to support them. If you make something good that users want, devs will typically support it of their own accord. And to be clear, Apple's policy does not require all apps to support Apple ID - only those which support another SSO solution like Google's or Facebook's. A malicious dev who wants to collect user data has no incentive to support SSOs in the first place. By not supporting SSOs, they can still collect user emails and abuse them however they'd like.