JH,
you are talking about IAD (Dulles), which does indeed have an underground passage connecting the main terminal (the second finest in the US imho) to the outer piers. They also had mobile lounges, but only to connect pax between the concourses -
afaik they have gone now.
I don't really see how the undeground spaces can register anything like as well as Saarinen's roof on the terminal itself!
Most airport terminals were built with legacy carriers in mind - and they have always preferred parking at a 'proper' gate with a jetway (air bridge). I don't think this model is compatible with the kind of turnaround you envisage, and also how much time / labour are we really saving? Pushback takes - 90 seconds?
At a low cost facility, I am with you - but I think the few that have been built have still used the standard jetway model, just without the jetways - so often using a single storey ground level terminal.
The problem is that an underground terminal would be hugely expensive, and that cost would outweigh the advantages in turnaround time. As for parallel runways, I have always felt that multiple runways, unless already there, are not compatible with the lo-cost model, given their huge cost to build and the likely need for extensive taxiing.