[goobynight]

Read the full context of that quote. There's even more in the SRE book.

"Don’t make your system overly reliable if you don’t intend to commit to it to being that reliable"

If a service has exceeded the reliability target for a given time period, you can take it down to basically let users know that this can happen and to not expect more.

You don't want them to get to the point where they are integrating so much with a service (and assuming a higher reliability that you have not promised ) that they end up mad at you when it performs worse, but still as intended, at a later date.