Conversely, sticking to pre-authored backstory speaks against trying to implement a principle like the one I just described. But in that case, the idea that players are free to set their own priorities for their PCs starts to lose its purchase - as
@Ovinomancer posted way upthread, that part of the GM's backstory which is feasibly available to the players given where their PCs are in the "sandbox" generates something like a list of options/setting elements for the players to engage with.
@Campbell upthread sketched a way of trying to split the difference -
story now in the streets, right to dream in the sheets - which is to say, apply the relevance principle in prep between sessions, but stick rigorously to prep during actual run-time and adjudication. But I'm not sure that still counts as a "living" sandbox, because the "life" isn't based on naturalistic extrapolation from prep plus the events of play.