cmd-B does nothing in a standard text field. If you mean text fields with code to handle keyboard shortcuts, you can just as easily have cmd-B surround text in asterisks as trigger wysiwyg bold formatting.
In rich text interfaces, keyboard shortcuts do more and work better than they do when typing Markdown in pure plain text environments.
That you can build better keyboard shortcuts for plain text areas is of course true, but it’s still a hack on top of a hack.