Well, as a Electrial Engineer who designs equipment that have a serial console, I think I give you several reasons that the serial console will be round for some years yet.
First, legacy. Most professional routers have come with a serial management console since ever. So anyone who's been trained to manage these devices use serial consoles for that. Of course, by being an IP equipment, you can manage them by accessing the same console using telnet, and you can upgrade their firmware using that console too. A USB to serial converter is a basic tool for anyone managing these type of routers
Second, design. In a microcontroller, one of the simplest devices you can use is the serial port. A lot of bootloaders for embedded devices (U-Boot, Redboot, CFE) usually start with a banner on the serial console even before configuring the RAM controller on the CPU, so you know your board is running and you can output valuable error messages very early on the boot processes. Other devices, even a true USB console, need much more complex drivers that are loaded later on the boot process or need more configuration options than a simple "115,8n1" somewhere on the manual.
Most domestic routers don't have a serial port. Well, they have, but you can't access them unless you open the case, the bottom line is that domestic users aren't even aware their wifi router have a serial port, much less that they have to use it. How often you need to unbrick your wifi router if you don't load custom firmware on it ? My guess is never.
Third is that it doesn't make a difference, as others have pointed out, if the equipment uses a USB to serial conververter, as the serial device will usually be limited to 115 kbps, even if your serial interface can transmit up to 12 mbps. Only CPUs with USB devices on them will benefit from a faster interface. The iMX line of processors from Freescale is one of them.