I would argue that password managers are not a "in every sense a really bad, bad idea" for a lot of reasons.
Let's look at password reuse for example. As soon as you have more than a few dozen logins, the possibilities are mostly either reusing one or few passwords, or writing them down.
Reusing is objectively bad, and for writing them down, the password manager makes it easy to use a really long and random password, which would make it tedious to write down.
Password managers makes the user life easier, at a big price if the master password gets compromised, as all the passwords get compromised at same time, in an unified way that by other methods would require much more specialization and effort for to gather together.
If that passwords are stored in internet even worst, one can take for sure those passw-managing servers are juicy targets, it is a countdown until the server will get compromised.
If the user is only storing the pass of chat forums, I think then is one thing the attacker probably will ignore, if the reverse engineering of one of those sites using user's name is not in the secondary target list.
Anyway, to use a unique password for every server, account, etc is a must do from the beginning of time, even for the temporal forum one had to register for to use a few minutes. It is the first computing directive. It's just the password managers are not accomplishing the objective of such directive.
Just because something is a "first computing directive", that doesn't mean people do it. If you're capable of remembering hundreds of high-entropy passwords, more power to you, but that approach doesn't work for most people. Password managers are better than reusing passwords between services, which in the real world, is the alternative.
I have hundreds of sites in my password database. Can you remember hundreds of random 32 character strings? I can't and I think there are very few humans who can
Let's look at password reuse for example. As soon as you have more than a few dozen logins, the possibilities are mostly either reusing one or few passwords, or writing them down.
Reusing is objectively bad, and for writing them down, the password manager makes it easy to use a really long and random password, which would make it tedious to write down.