follow-linkcondition, so that the link obeys mouse-1-click-follows-link. See Clickable Text. See Buttons, for an easy method of implementing such clickable links.
Changing all the Emacs major modes to respect this convention was a lot of work; abandoning this convention would make that work go to waste, and inconvenience users. Please comply with it.
The reason for this rule is that a non-prefix binding for ESC in any context prevents recognition of escape sequences as function keys in that context.
For a state that accepts ordinary Emacs commands, or more generally any kind of state in which ESC followed by a function key or arrow key is potentially meaningful, then you must not define ESC ESC, since that would preclude recognizing an escape sequence after ESC. In these states, you should define ESC ESC ESC as the way to escape. Otherwise, define ESC ESC instead.