terminals were once real physical devices for sending input and receiving output from a mainframe computer. The shell was not a real physical device from the start, but the same what it is today, a command interpreter, a processes running inside the mainframe computers' ram. Why would one even consider to turn those physical devices into processes which run alongside the shell in the ram?Basically we evolved from:physical input/output device and computertophysical input/output device (keyboard/monitor) and emulated formerphysical input/output device (terminal) and computerWhy?How does it make any sense to evolve from dualism to a kind of inbetween dualism and trialism (if the word exists)?
Why would one need a terminal emulator in the first place?
terminal
As kusalananda says in the comments, we still use terminal emulators because we don't have an alternative.This trialism is observed only in unix based systems. You can live on a windows machine for years without ever opening a dos prompt. I guess the reason terminal emulators are still a thing today(in unices at least) is because we haven't found a better way to use the shell interactively. And shell is a very useful part of unix based os's. You can't program your mouse to do stuff you find yourself doing again and again(well you can, but it's not nearly as elegant as the shell).So, the terminal emulators will be around till the day we find a better way to communicate with the shell, or an alternative to the shell itself.