The disposability has to be factored into the environmental impact.
Most MGs have been in the landfill for decades. It wouldn't be a surprise if this car had been sitting on blocks in a garage for decades. It's disingenuous to imply this car is still running for any reason other than because it has an owner that both wants to restore it and has the ability to do so.
I suspect once its restored there's a fair chance that it'll park in a proper garage and be driven a couple of times a month on nice days during summer until the days start cooling off and it gets stored for winter.