They are welcome to get a regular patent covering their new approaches to implementing thin hardware (e.g. new battery technology or whatever).
Getting on a monopoly on (or due to) the design trait "thin", however, is not acceptable, even if it were the case that Apple came up with the idea of thin devices, which is obviously not so.
I already said it was the overall design aesthetic, not one simple element, but that it's obvious that Apple is a leading innovator in the space of thin industrial design. iPhone, iPod, iMac, the Air, are all some of the thinest in class. Plus you can see how even the CrunchPad/JooJoo changed it's design aesthetic to match that of the iPad after the iPad was publicly announced.
And where/when did monopoly come up in this conversation? To my knowledge patents do not enable a company to a monopoly.
Getting on a monopoly on (or due to) the design trait "thin", however, is not acceptable, even if it were the case that Apple came up with the idea of thin devices, which is obviously not so.