re 1.: Support the 10 most used email providers and you will probably cover 99% of users. Most people have a Gmail even if not as their primary email, but so th

re 1.: Support the 10 most used email providers and you will probably cover 99% of users. Most people have a Gmail even if not as their primary email, but so they can use various Google services on Android, or an Apple ID to do the same on an iPhone. Unless you have data that suggests differently, I really don't think this is a valid issue nowadays anymore. re 2.: If you don't want to use your private email for everything, then you already must have another secondary email, because there is literally no registration process which you can get past without an email. So just use the email which you find appropriate for a given website. re 3.: Same as 2. You must have any sort of email to register anyway, so whether it's your primary email or your secondary "anonymous" email, it's still the same principle. re 4.: Not doing something smart today, because there could be potentially a futuristic abuse scenario in some point in the future is not smart. Do what's best today, if tomorrow all email providers turn evil, then re-introduce your own login system, but you'll still need these email providers to verify your users.. so not sure how far you will get... but anyways.. this is a non-existing problem today and probably never going to happen. So why degrade experience and security for a Utopian future? So yeah... I much rather use a secure and convenient auth system, because it has way too many advantages and we already rely on it, we just don't admit it. Might as well go the full length to get the best experience from it.