Shops do change, baboonery. Demographics change, populations move, types of shop come and go. Under your argument, shopping areas would never change, improve or grow because 'if they were going to, they'd have done so already?'
Indeed so. But the proliferation of high-end delis and the like in nearby areas and other similar areas of London in recent years suggests that they'd be here by now if there was a market. Whatever happened to Organic Republic? And that had passing trade, which the Mercury site wouldn't have.
There's too much pie in the sky on this forum about Forest Hill becoming some sort of middle-class utopia (a child-friendly one, of course). Green and Blue on Perry Vale? Just take a minute to think about how ridiculous that is.