What is left unchanged?
Stationarity, as an assumed state, becomes questionable as soon as gravitational circumstances become more complex, as well as in the examination of other systems, such as in the value relations of economic systems or in the epigenetics of social relations, educational or influencing methods, that is in the effects altering hereditary genetic properties.
Stationarity is a simplification tool in modelling, just as the linearised approach to non-linear processes. All this requires thorough epistemological and practical considerations. Simplification techniques are smart tools in science and technology, and their assessment and completion with more realistic models are the usual labour-intensive path of progress.