Earlier quoted context omitted.
>You should always paste into an editor first and that editor should not be running "inside" a terminal -- or rather I don't understand why those that like interacting with their editor via a terminal interface don't create a new protocol that doesn't have the warts and vulnerabilities the current ancient protocol has.
Compatibility, basically. e.g. Vim creating their own protocol is rather pointless if no terminal implements it, and some terminal creating their own protocol is also rather pointless without application support. Plus there's tons of software out there and it'll take a long time for it all to be rewritten, if that ever happens. e.g. X11 will still be around for a long time as well. So you will need to remain compatib…
The hairiest Emacs Lisp code I know about is the code for responding to mouse actions, e.g., clicking and dragging, and most Vim users don't even want to use a pointing device to interact with Vim IIUC. (In contrast, 80% of Emacs users who responded to a survey do not interact with Emacs through a terminal interface.)
For example, the old scripting language Tcl has its own GUI toolkit named Tk that has a pretty good reputation.
>And all things considered, it's not that bad
It seems bad enough to me to be worth re-architecting.