Would concatenating (i.e. joining together into one string of text) a secure (i.e. Diceware) password and other, much less random, passwords be a good idea? This new password would be used as master password for my password manager, and, since it would replace all my passwords, why not concatenate all of them into one and append a secure one? Obviously, each service would use a new password generated by the password manager.
Intuitively, it would be more secure since the password is longer. The disadvantage, of course, would be that I'd have to type more.
Are there any downsides I'm not aware of?