The password manager being 'your brain' implies that you have only a hand-full of passwords... my brain has no way of remembering 1000+ passwords and logins, while I prefer to use random usernames and passwords. Sure, I don't need 2fa for a knitting forum, but I still need something to remember my login and password... try to never re-use either.
I have two categories of passwords. One for knitting forum class, and one for password manager class.
No, it isn't a crisis if someone gets my credentials to the knitting forum, the pics of acrons forum, and the local 'reserve space at the county pool' website... all in one go.
I can just change them all at once, from the letter 'a' to the letter 'b'.
To further support your argument, I'd suggest that requiring a login at all for this class of "service" is bullshit.
I've set the bar pretty low these days for "if you require a login, I go somewhere else" because there are plenty of places that just don't need it in my opinion.
To be honest, I class Twitter as one of those places; I go there to read certain information from specific "outlets" but Twitter as we all know have made it very difficult (as with other social sites of this type) to be read-only.