You asked about the control key. Have you ever used "control-c" or "control-d" from a Unix command line?

Yeah. Of course. And that's what I was referring to. There is no reason for a separate cmd key to exist. Some stuff is switched to cmd (copy, paste, select all) and other stuff stays with ctrl (ctrl+c, ctrl+d). See now I can't even remember how to close a tab in chrome. Is that cmd+w or ctrl+w? All of this is really arbitrary.

Yes, but you originally said there was no reason for a control key to exist.

The modifiers serve two different purposes: control+something is sent to the remote host, cmd+something is handled by the application.

So if I run a long running grep and terminate it using Ctrl+c, which remote host am I sending the command to?

It would be sent to whatever process is attached to the current pseudo terminal. (When I said "remote" I was more referring to the SSH scenario, but the mechanism is the same...)

Ok, so that means the remote vs local logic doesn't apply. In fact none does. The demarcations are arbitrary. Cmd is randomly used and ctrl is randomly used. Even if there is a remote vs local rule, why not make ctrl do something for local programs? Do you use a separate keyboard for your ssh and another one for email? No. The keys switch contexts according to the active program.