Who is going to discover it in that time? Not the maintainers, they've already released it. Their window for scrutiny has passed.
There is some sense in giving the early adopters some time to raise the alarm and opting into late adoption, but isn't that better handled by defensive use of semantic versioning?
Consider the xzutils backdoor. It was introduced a month before it was discovered, and it was discovered by a user.
If that user had waited a few days, it would just have been discovered a few days later, during which time it may have been added to an even wider scope of downstream packages. That is, supposing they didn't apply reduced scrutiny due to their perception that it was safe due to the soak period.
Its not nothing, but its susceptible to creating a false sense of security.