You're missing the issue of connectivity. Yes, ACARS is automatic. But it TRANSMITS on VHF and/or satcom. If it cannot connect to a valid radio signal or satellite, nothing is going I/O. Think of it as being in a dead zone with your mobile phone. Without bars, you're not making calls or texting. There are dead zones in the ARINC network that ACARS uses.
But unless theres a solar flare then satcom should be 99.99% available. Its like the GPS satalites, theres only an issue when driving in large cities like NYC or in tunnels.