And I can tell you that a "decent" wage according to Western standards is NOT 4000 - 5000 Yuan RMB a month;

You can tell me what you have stated above till you're blue in the face, but I beg to differ.
I guess you have not analysed the situation in China re the local cost of living relative to the level of foreign teachers' salaries in depth. Neither have you taken into account the relationship between the number of hours worked by foreign teachers and the total renumeration received by them after tax.Being paid for doing nothing in one's holidays is a good deal.
Currency is only paper. It only assumes value when it is spent. As the cost of living is far cheaper here in China than in the West, a person on a monthly salary of 4000 Yuan can live rather well here. In fact, a lifestyle supported by such an income is definitely middle-class in the metropolitan areas of China belonging to the less developed parts of the country, which are not few in number.
Please do not make sweeping statements from afar, like the one you have made above, again. Those here in China are 'in the know', and hence are in the best position to tell potential newbies the true situation re teachers' salaries at this time. They should be listened to.