The "one password flaw" has never been an issue, but my bank uses proper two-factor authentication with a physical card-reading device, so maybe that's why... I've never actually encountered a website that places problematic restrictions on passwords except (weirdly) Microsoft.
But they're just personal anecdotes that those flaws haven't been an issue for me, but I agree they exist and could be show-stoppers for others. I certainly wouldn't recommend it to anyone non-tech-literate. If I did need secure storage outside of that system (which, you're right, does happen–mostly for wifi passwords and the like) then I just use the system keychain as intended.
But I do still have concerns about the overall security of the system simply because I don't understand it well enough...
> Once you have that secure storage, why not just use it instead, since it can resolve nearly all of the problems?
Because I don't want to pay for 1Password licenses, or be caught out if I'm using someone else's computer, or if all my backups catastrophically fail :)