Underbody Shield problems of the 1999 Infiniti I30

I took my well-maintained, garaged 1999 Infiniti I30 to the mechanic after noticing the handling getting sloppy and severe inner tire wear. Upon inspection, he informed me that the front core support (the box beam frame member that extends the width of the car and supports the radiator) was rusted through. The corrosion is severe to the point that the sides of the beam are eaten away over 1/3 to 1/2 the length. In places, all that is left supporting the radiator is the top plate. The center support bar that holds the front end of the motor in place is supposed to be bolted to the bottom of the beam, but because of the corrosion, it is now hanging cantilevered. The car is not safe to drive with the motor inadequately supported. Relative to the rest of the car, the severe corrosion is localized to a 2-3 foot section of the frame (as if there is something causing the corrosion to accelerate in this particular area). Properly manufactured structural steel should not disintegrate like this.