There's always a window where both will be in use, because we can't synchronously replace every running process everywhere (not that it's even a good idea without a canary). The shorter you try to make that window, the more needless pain is created and plans disrupted. While we could use prod to beta test every single version of everything, that shouldn't be our priority.
In this case you'll have a very short transition, between all consumers updating their client code (possibly in a backwards compatible way) and the change in the implicated system being deployed, not the other way around.
Introduce a breaking change into a common library and now you have to update every other dependency to support it.
Not so bad in a monorepo. But when your codebases are distributed?
