Again a simplified answer from my side (being no engineer, but a retired fighter jock): The airflow through the engines has to stay subsonic. Therefore the engine inlets have variable ramps, which control the airflow to the engines and reduce the speed of it to subsonic. That again heats up the air and that was the limiting speed factor in the RF-4E. "duct Temp high warning".
It might be different in other jets, but in most of them the pure structural limit is not a factor.
franzl