Alice installs malicious eslint.
    Attacker has control of Alice's machine and signs a malicious version of Alice's package using Alice's key.
    Attacker publishes a new version of Alice's package.
    Bob installs Alice's (malicious) package.
    Attacker has control of Bob's machine and signs a malicious version of Bob's package using Bob's key.
    Attacker publishes a new version of Bob's package.
    ...repeat

IMO signing could only work if it were combined with 2FA so that each time a package is signed, the signing operation is verified by the user on a second device. Otherwise it doesn't really impede a worm significantly because the nature of this attack is that the maintainer's primary device gets completely owned. We are all very lucky that the eslint attack only stole npm credentials instead of installing rootkits.