How can I get 1-10MB/s of debugging data to and from a dev board?
Modern successor for a RS232 communication port is supposed to be USB. However, people need to realize that if it is used for debug purposes, it must have some serious architecture support at system level, the port must have some internal bridges to uP registers and memories. 

Starting in 2002, Intel has made an attempt to implement mandatory debug functions into open-standard EHCI controller. There was even some support for it, from Linux and Microsoft side, although, as I understand, all the effort had limited success.

With an advent of USB 3, the effort was resurrected, see USB 3.1 Debug Class Specifications.

Most recent Type-C connector also defines "Debug Accessory Mode", see pp 59-60. I believe there are several auxillary verions in the works on how to use the pins in alternate way, including JTAG functionality.

However, the old RS232 is not going away and refuses to die. The reason is that most uP and mini-OS are relying on venerable COM ports for many functions, and all Linux/Android flavors have a built-in debug support in kernel. And the UART circuitry/port is fairly simple to implement in silicon with very few resources. However, instead of placing bulky DB-9 connectors (which sometimes are bigger than the device itself) people are embedding a UART-USB converter on-board (typically of FTDI type), and use the miniature u-A/B USB connectors to attach to debug host, emulating COM port and using standard terminal software to access it.