I completely disagree. The ability for serial to go over POTS makes it ridiculously cheap compared to building a reliable ethernet connection over hundreds or thousands of miles.
This is identical to ethernet. You need external device then, dial-up modem or CPE, no difference.
The separate part is what makes it useful. The only reason you should need to access the serial port is because the network is not functioning. If you move the last resort access to be network, how do you access it when you have network issues?
Or because device is not functioning, and if OS is broken, so is your 'OOB'. Or maybe you fucked up upgrade and corrupted image? No way to recover over RS232 (RS232 image upload not supported anymore, even if it would be, Juniper is hard set to 9600bps, even if it would support 115200 it would take 12h to upload image, faster to fly with usb key).
But having a console->serial is significantly less complex than console->IP_Stack->ethernet. So many more things to go wrong. I've never had a device that had a faulty serial port. I have seen numerous faulty or misbehaving network ports.
Only thing matters is that it fails at different time to production network.
I like the current trend of vendors like Juniper. Dedicated management ethernet, *and* serial console port. Best of both worlds.
This is fully on-band port ethernet-port, relies 100% on the host OS being up and running. Replace this port with true OOB ethernet, keep RS232 for people who can't migrate day1. You never introduce new thing and kill old thing day1, this is not how things work. You add new thing, then after good amount of time phase-out old thing. -- ++ytti