When previewing a changeset on the frontend and the Admin Bar is present, in addition to there being a Customize link to allow you to enter the customizer to make changes, there could also be a Publish button that would allow you to make the changes live right there. This would tie into a frontend editing experience for the customizer, where a user could start a frontend customizer session by clicking Customize which could then bootstrap customize-preview.js and start a new changeset, with inline editing of elements, floating controls, heavily relying on selective refresh (exclusively) to make changes. A user could then navigate around the site on the frontend as normal, with the changeset state persisting, and then once they are happy with the changes they can go live. Anyway, adding a Publish link/button to the admin bar is one part of this.