This file could define the accepted password format. Password managers could retrieve that file and know exactly how to generate a password.
I can order a pizza via Twitter, but I'm still using passwords?
Instead they could create a "password complexity requirement specification" to allow sites to embed a universally machine readable description of their requirements, which all password managers and users could benefit from. I don't know if any such effort exists already.
Many ordinary people have no idea whatsoever of the relationships between clients and servers, and less still of that between different clients (what is a web app? Is gmail my search engine?). Throw in an interposing piece of software that not only manages but duplicates login credentials for these various mysterious entities, and they are completely and utterly despondent, hopeless and lost.
Their VPN software is apparently very easy to use (from reddit comments). The same will probably apply to the password manager. They also seem to have a good marketing and PR team since I see them around quite a bit in youtube videos (Linus' being the most prominent one) and ads thinly veiled as articles. These two points alone have the ability to make the difficult mainstream user market to use it.
If all your security tools had similar UX and only a single login (what we see as single point of failure is usually seen as convenience), then you'll get many people to use it.
I use it for very basic things, so I guess I wouldn't know.