Yeah, SSO does not seem to be a problem inside e.g. corporations. The SSO solutions I've encountered there work rather well, and nowadays employees authenticate not much more than once or twice a day to get access to everything.
SSO outside of such closed realms, i.e. in the open Internet, rather only seems to be a "problem" because people don't want to adopt it. Instead, password managers are the solution that won. The Twitter thread mentions "No sweat: you can have as many Eth accounts as you want w/ different ENS names", but if you end up having one (or more, see throwaway accounts on Hacker News) accounts for each site anyway, why go through the trouble with public/private keys, when the random shared secret in your Password Manager works so well?