If something got a look inside your book, everything is compromised. At least by changing to another random gibberish password, an attacker would not get back in (in contrast to someone that increments a number, for example), but how much damage can something do in that time before you notice? How long do you think it takes a skilled attacker to install a backdoor (the answer is definitely less than 44 days)?
This is the second major problem with password rotation: it is false security. If credentials are compromised, immediately change them: don't wait 45 or 90 days.
If they're not compromised, changing them is basically pointless, because humans are going to human and either have an incrementing number, predictable pattern, or actual complex, random, unrelated passwords ... that they then write down.
Writing passwords down on paper and leaving that paper in an insecure area isn't.
Your little address book full of passwords is the same as a password manager, and for a lot of people it's easier for them to manage the security of a booklet than it is to handle a password manager properly. And for those people, it's definitely worth using a book for the sake reasons we say using a password manager is a good idea
I used a book for passwords issued by that employer because at that time they lacked any in-house password manager and I foresaw from the outset that I would want to be able to hand them "my passwords" in the limited context of that employment as distinct from stuff like my personal Wikipedia account, my Google account and so on.
Personally I would be much more concerned about other things - my stuff is there, not to mention often myself - but I'm sure that a vast faceless corporation would be primarily concerned with the potential for these hypothetical bad guys to learn and maybe disseminate secret passwords.
How often do you think that actually happens? It's a bit... high risk for opportunistic "cyber criminals" don't you think? Figure out where key employees live, then break in and... look for passwords they've written down and then presumably use them quickly before they're invalidated.
Besides, actually remembering your passwords is a security flaw, too. Memorable passwords are naturally less random, and if the password is in my mental memory, an attacker can compromise my password by smashing me over the kneecap with a heavy wrench until I tell them. If I don’t even know my password, that’s probably bad news for my kneecaps but slightly more secure overall.
If someone did break into my house as an act of espionage (corporate or nation state), I'd be worried about much more than just my password security.
However, I would also not defend mandatory password change policies, so you would never end up asking me :D
† I can't tell you now because the day after I ceased working for them I sent the nice leather-bound book I write passwords down in to my Product Owner, so that the answer to all subsequent "Do you happen to remember the password for...?" questions would be "Either it's in the book which you have or No". To be fair he only called me and asked that question once, which is why he was my favourite Product Owner, "capacity to learn from experience: 10/10".