It's a stretch to describe road debris hitting the undercarriage of a car a "collision". You should expect a vehicle to be engineered so that small objects hitting the undercarriage don't cause significant damage.

Making subassemblies replaceable in part rather than in whole is definitely a bonus. When they're only replaceable in whole, I do consider it deficient engineering, especially if a small change would make it possible.

An example from motorbikes: I would never buy a Triumph Tiger 800 because the rear subframe is welded to the rest of the frame. Any drop which causes a bend in the rear subframe - easily enough done since the passenger footpegs and exhaust are attached to it - would require replacing the whole frame to fix. In most high performance bikes, the rear subframe is bolted on.