The reason one cannot push the ramjet any higher is the air, slowed to subsonic speed, is simply too hot. If it's sufficiently hot the materials of the engine

The reason one cannot push the ramjet any higher is the air, slowed to subsonic speed, is simply too hot. If it's sufficiently hot the materials of the engine cannot withstand it, and even combustion stops being effective (because the temperature drives reactions in reverse, preventing heat production.)