Ignore the haters, they don't understand the politics for this. I used to design industrial Ethernet networks for a large vendor, and we spent quite a bit of time pointing out to customers how dangerous the direct lines were. However, IT departments have very little say over manufacturing networks. This isn't always a bad thing (see the many IT/help desk horror stories). Because the remote access is often required as part of the maintenance contract, offer to partner with manufacturing to install a small firewall with access filters that are controlled by IT, but set (requested) by manufacturing.
A small Cisco ASA, Juniper SRX or its like will do the job nicely, and can shield you from hack attempts along that access path.