The common assumption of stationarity is at odds with the unpredictably changeable nature of the real world. This is not to say that all the analytical efforts of the past and the progress achieved should be disregarded.
It means rather that in order to make the best out of them one needs to remember that models are by necessity “local” approximations to very complex phenomena and they should be used with sound practical judgment as a framework, not a straightjacket, for our decision-making.