All about risks, not just the fact the device is RS-232 only, but can the manufacturer support the equipment and if it fails whats the cost to the business while the machine is down.
Sure we can all get 486's with ISA cards if the device needs connectivity to the outside world, but the device of that is a business risk and needs Mgmt to be aware of the issue.
I've had a parts carousel system go down for weeks while we replace gearboxes, made worse by a switch from AC to DC by the manufacturer. Costs to the business were huge and all production was slowed down. Mgmt were informed of the risks but wouldnt spend any money till if broke.
Basically you're looking at a CYA situation, make sure mgmt are informed in writing (email etc) of the issues around the machines and what costs there are vs getting in new machines and added benefits of new machines