It's exactly what the car should do - the safety cell (where the driver is) should remain intact, and did, but the rest of the car should break into as many pieces as possible to absorb the impact. Especially at a speed like that.
That's not entirely true. In fact, I'm going to say it's not. In the old days, that's what carbon fibre cars did, but not today. Today's cars have a monocoque chassis (you've called it the safety cell) that the driver sits in. There are front, rear and side crash structures (nose, rear crash and SIS tubes) which must absorb an impact in a controlled way and side intrusion panels to protect the driver. In the pre-season crash tests for homologation, not only must a certain maximum deceleration on the driver not be exceeded from a particular speed, the chassis must not break when these are tested. That includes not breaking the gearbox (because the rear crash is on the back of it) and certainly not the engine breaking off the back.