Em man apt-get
:
-y, --yes, --assume-yes Automatic yes to prompts; assume "yes" as answer to all prompts and run non-interactively. If an undesirable situation, such as changing a held package, trying to install a unauthenticated package or removing an essential package occurs then apt-get will abort. Configuration Item: APT::Get::Assume-Yes.
Você precisa de --force-yes
para aceitar esse aviso, IIRC.