It is a question what a plane is designed to do. As mentioned, a blunt airliner would not fit the fuselage and wings behind the generated shockwave. It would also create a normal shockwave significantly in front of the nose, and not an oblique shock at the nose, which would result in far more energy being lost and hence drag. Furthermore, a subsonic airliner flying supersonically is not easily controllable. It is not designed to be controllable at supersonic speeds. The loads on the airframe would be higher, but for a short period of time it is survivable (eg the China Airlines 747SP, or the new Bombardier regional Jet, both of which survived supersonic flight for a short time - but as far as I can remember, the 747 suffered structural deformations). In general, a subsonic airliner accidentally going supersonic will be very hard to control, and it will quickly lose velocity due to the substantial increase in drag, and become subsonic again.
What is different about Concorde? The aerodynamic shape. The materials - it is far more heat-resistant. It is optimised for supersonic flight at the design stage.