I think you dramatically overestimate how many people can do this consistently for years on end. People who don't use a password manager simply reuse passwords. That's what they've always done. Any solution that ignores this observation is doomed to fail.
I disagree, since I think it has more to do with lack of education that this is something you can do; have a simple deterministic algorithm for generating passwords based on your service. It's far more convenient than a password manager, and can avoid the single-point-of-failure.
I think you dramatically overestimate how many people can do this consistently for years on end. People who don't use a password manager simply reuse passwords. That's what they've always done. Any solution that ignores this observation is doomed to fail.