The connector is gone, but the need for something equivalent persists. Network, adapters etc are nice, but they are very complicated to use; complicated enough to require a device driver [stack], which implies a booted operating system.
Until the OS is booted, all those ports are dark, IOW, one cannot use them for debugging the boot process, or the (booting) loader and kernel. The IBM PC, as much as I despise it, makes using the serial port trivial, since the BIOS effectively has a device driver for it (although manually driving it isn't much of a big deal either).
mov ah 1
mov al <char>
mov dx 0
to vomit <char> out the serial port from 16-bit real mode (i.e. the mode the loader starts in)
So one way or another, a serial port (equivalent) will persist. It might get a little harder to access, though (e.g. some Android phones have their serial console going out the audio jack...), but it can't be done away with altogether.