Sorry to come to this thread so late. Surely we do have TCP/IP to serial bridges?
TCP/IP follows the ISO layer standards. Serial comms (RS-232 and RS-422) follow earlier monloithic standards. A bridge is therefore necessary for the two different standards to communicate. Typical application of a TCP/IP to serial bridge is to connect old display signs (programmable via serial) to a modern network.
Alas they don't help with this problem.