Ok, lets see. An engine for an airliner has several shafts, titanium, and huge titanium fan-wheels, steel gears, pumps, starters generators etc, etc.. these are virtually indestructable.
The central mass of an airliner fuselage is aluminum.. and not concentrated it is 200 feet long, by far the greatest weight is the wing spar and fuel lines and wiring.. all of these items are very fragile when smashed into a solid like a concrete wall..
The engines and landing gear are NOT fragile. they are hardened steel and titanium. they will survive where aluminum will just melt or crumple.
Think of a bullet, lets say a 180grain 30 caliber copper jacketed bullet.. it's mass is concentrated in a single lump.. it is capable of pennetrating a lot of steel plate, a tree, etc. Take the same 180 grains of copper, make a 5"x5" sheet out of it, [roll it thin] and to the same with the lead core of the bullet.. put these thin, fragile sheets of metal that are very lethal at 3100fps when fired from a rifle, and try to do damage with the thin sheets to anything, even if you could enclose them in a vessel and move the sheets at 3100fps, they would not do much if any damage when hitting a similar target.. the mass is too spread out..
An airplane has it's 'red-line' airspeed because various parts of the plane do have supersonic airflow around them when the redline is exceeded. if the plane is pushed past the redline, parts of the airplane experience supersonic flight, airflow and the resulting stresses, even at say 425 mph indicated at sea level, parts would be damages or go 'missing' from the plane.. This is why developing supersonic flight was such a job after WWII, and one of the big problems with WWII planes,, they could reach Mach 1 or near it in a powered dive, but the airframe would come apart..
The Japanese planes could not reach Mach 1 when diving as a Kamakasi.. they were drag-limited and didn't have enough power either. But the US P51 could, as well as the P38 lightning.. read up on it.. there was a HUGE effort after WWII to test and develope an airframe that could be flown and controlled above mach 1..