> In contrast to the problem you've stated, if I were to exclusively use my Google account to log into websites, it becomes a single point of failure if the service was down, and if it were to be compromised.
Very true. Sadly there's no real right or wrong answer here; a single point of failure but a better secured portal, or a decentralised network with arguably less secured portals.
Personally I try to use a balance of both: Twitter passports for sites I don't trust and passwords for sites I do trust. But that's just my personal preference.