The idea is that if you destroy the ecological base, economic activity will cease. The environment will be unable to sustain economic activity and, therefore, life. The productive base of the earth will be constantly shrinking, while the global population, left unchecked, will continue to grow. Much more common is the destruction of the productive base through pollution, e.g. the gross destruction of the productive base in and around the Aral Sea, due to the ravages of pesticide-intensive cotton production. Sustainable development still contained the idea of economic growth.
Activities are sustainable which protect and enhance the ecological base of production.
When activities are unsustainable, they require measures to protect the environment, in other words, to turn traditional centres of production into environmentally friendly workplaces. So we need environmental industries to control, mitigate or limit the environmental damage of industrial and consumer practices. Even at their best, some industries will be more sustainable than others. There will always be a need, for example, for mining and smelting. These activities have to be made as safe for human beings as possible, and the least environmentally damaging as possible - including the work environment of those in the mining industry.