So, you see Story Now as always involving immediate consequences and a tight loop of feedback and increasing tension? I'm not so sure... So, for instance, here's a scenario drawn from past experience: The Space Station is doomed. There is NO way off, everyone on board will perish at the conclusion of the campaign. Now, clearly the main driver of tension is FAR OFF (in relative terms of scale, far off in that it is many many scene frames down the road, IMHO it isn't relevant as to how many hours/days/whatever this represents in fiction).
Now, obviously this game can ALSO feature as tight a loop of ongoing action and feedback/consequences/immediately generated tension. It can also include other medium term considerations, perhaps. The point is, I think the PREMISE ALONE made it a type of Story Now scenario. Everything the PCs did or experienced was always fully in the light of "and now I will die." It colored the whole experience, heavily.
So I would propose that things like DW's fronts/dooms likewise are an element of Story Now. I don't think they necessarily MUST exist, definitionally for it to be Story Now, but I think they may be sufficient to at least imbue a game with a significant element of Story Now character. I think that might play a role in higher level TB2 perhaps going in the SN direction as well, the PCs claw their way towards real success, they reach 7th+ level where they are clearly going to be movers and shakers (albeit perhaps also outsiders). At that point IMHO the most natural path for the game to follow is that the Final Doom of civilization, TB2's 'Ragnarok' (or at least an overwhelming disaster) raises its ugly head over everything. While the ultimately doomed nature of civilization is pretty much backgrounded in normal play, and thus not a big factor, it seems like it is always a tool the GM can turn to in order to really dial up the pressure at a certain point. This also jibes with the observation about the high level BitD game where eventually things inevitably turn to existential threats to Duskvol.