Something like an RB2011 (which would be plenty fast enough) but with 6 to 12 rs232-over-RJ45 ports instead of all the ethernet ports. -48v input would be a nice bonus, like on the RB1100AHx4.

ROS already has everything needed to be a console server, plus lots of things that console servers from the usual suspects do not, and I'd imagine could sell for a much more reasonable price ($2000 for 16 serial ports is *not* reasonable!)

I don't know if there is a sustainable Market for Console-Server (RS232, RS422, RS485, etc..)
Most new Devices today have Ethernet or Wireless-Connectivity...

Nevertheless,
If you are looking for a Solution TODAY, look up W&T (https://www.wut.de/)
I used them all the time for Indutrial application like,
- Parking-Gates
- Check-In / Check-Out Systems
- Industrial Equipment (Building air conditioning)
- Automation, etc...

I highly recommend their product!
Never had a Device fail on me, only the Power-Supply's =)

This is actually not too common requirement so the market is small, therefore amount of sold units small as well. That means the price per unit will be high no matter who makes it.

However, I believe there is a cheap solution! Many routerboards come with USB. I know for sure that it supports at least some (but likely most) USB-to-RS232 adapters. You know - those typical gray/cyan cables, often marked as "cisco console cable" and sold for few $ on ebay.
It came into my mind just now, but I believe if you plug an USB hub into routerboard, you should be able to connect multiple of those adapters, therefore getting many (possibly up to 127?) RS232 ports per routerboard.

EDIT: yep, it works. I plugged both my USB-Serial adapters via a USB hub. Both connected, both working at the same time:
With this approach, I am confident you can create almost unlimited amount of consoles for just few $ per console...