Earlier quoted context omitted.
I thought the point of a password manager is to allow you to have separate passwords for numerous services which are protected by centralizing those passwords somewhere (preferably somewhere you control, such as your own phone) and protecting the result with one password. Your model of them seems to involve them replacing a password locally, which I don't think anyone would recommend.
I would say password reuse can be pretty good! Simply have your own rule such as "letters 2 and 5 of the domain name" and combine those with your reusable password. In fact, I'd go further and say that you can do this with your login name. So for example: myemail+by@gmail.com for eBaY This also helps mitigate those attacks where the attacker actually contacts support and socially engineers them into giving all your i…
1) If your password is cracked, a hacker seeing it might deduce your algorithm and then deduce that your Amazon password would be 'myemail+mo', etc. Now your scheme is out the window.
2) Even assuming your password is salted and hashed securely such that it can't be decrypted, Ebay would probably reset it as a precaution. So now you've got to change this password. Maybe you decide to do the 3rd and 5th letters of the domain name instead. So now your scheme is 2nd-and-5th for all but Ebay, and you've got to remember that the Ebay one is unique. For now. As more leaks or resets occur down the line, however, you'll have more exceptions, to the point that you'll be forced to maintain a list of what KIND of passwords each site uses. In which case, why not just use a password manager?