As I understand, the problem with anything over Mach 4 (or 5), the metal begins to disintegrate. It is fine for a one way missile, but not a reusuable aircraft. Without some alien tech (see 1990s game "X-Com 2"), I cannot believe that we can build a reusuable aircraft that can reliably and safely fly for long periods over Mach 5.

Ramjets are limited in the speed they can operate because slowing the air to subsonic speed in the engine causes it to become hot, and the temperature increases rapidly with speed (the kinetic energy of an incoming air parcel is proportional to the square of the vehicle's speed, and most of that energy is being converted to heat.)