There's been a lot on the news lately about the huge proportion of cars that are sold on PCP deals, which are normally 3 or 4 years long. Speaking with a well known German manufacturer's dealer lately, they said around 90% of sales are now achieved with this type of funding.
Now, with this in mind, manufacturers don't really need underbody components to last longer than maybe 5 years, as once a car has changed hands, they tend to be very reluctant to assist in situations like this, as their perceived liability 'fades' with each new owner, especially if sold on through a non-franchise agent.
But it's not just MG that are cost-cutting on underbody protection - I saw an '06 plate Hyundai Tucson in my local breakers earlier this year, with rust holes in the rear subframe that you could poke your fingers into...
Having only done 70-80k (and a diesel), the engine was being removed to send to Poland, whilst the exterior body was near perfect. Yet underneath it looked like it had been parked in the sea...