What would you do...

In this day and age, what are the possible pro's to speccing an upgrade of an entire plant with Serial (RS232) comms to field devices?

Note: Both field devices and PLC's have onboard ethernet. Making the RS232 comms possible requires additional hardware.

I would point out the superior ethernet capabilitiy but if they want to pay more why not take the work/ money anyway

Can you clarify the existing plant configuration and the purpose of the upgrade.