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If someone has access to your user account and you're running tmux, they can just attach that tmux, they don't need to mess around with send-keys.

If send-keys didn't exist, a script could attach tmux in its own pty instead.

Detach/attach is a fundamental part of how tmux works, everyone must understand that it is fully available to anyone with access to your user.

GNU screen also has a similar feature to send-keys (they call it "stuff").

It has always been good practice to detach long-running root programs by starting a new tmux as root rather than running them in a non-root tmux, and to use sudo rather than su to run root commands inside tmux the same way as you would outside.