I dont think the main concern here is that the new part would be welded in. When the car is in design process, each weld is carefully calculated to provide the

I dont think the main concern here is that the new part would be welded in. When the car is in design process, each weld is carefully calculated to provide the exact binding force at the precise location. In case of accident, the car is modeled to crumble just right to absorb maximum energy without compromising cabin space, thus protecting occupants. The process of cutting out and welding in parts almost always means altering the stiffness of such bonds (and/or moving the bonds to different locations), which can be very dangerous in the event of accident