It's a really good question to ask "who wants Antares/NGL" and "why do they want Antares/NGL".
Originally Antares, like F9 and K-1 before that, were funded under a non-EELV program to provide Delta II class launch services, as Delta II was (still is) going away, largely due to its high labor costs (and little crossover with other LV's).
NGL isn't a Delta II class but an EELV class. If Atlas V flies indefinitely (which is possible but not assured), apparently the Delta II class need that handled Cygnus payloads ... seems to be obviated by a low cost, higher volume Atlas V. And, as we've seen, no additional missions on the manifest. At a guess, the theory here is possibly that a RTLS 24 hour F9R with a sequence of F9US probably handles the rest of that need below Antares costing.
So the only remaining reason is for OA to self-contract/manufacture/qualify/launch entire missions as a one-stop shop - which we have yet to see them do, kind of like Boeing/LockMart do through ULA themselves (haven't seen SX do this either yet). How likely is that?
As far as NPO Energomash, does it really make a difference selling one RD-180 or two RD-181's? Same either way (if Atlas V still flies post Vulcan step up).
NGL, like SLS, exercises the solids, in this case in the potential performance of NSS missions. If SLS, then lowers aggregate costs due to volume (Antares/NGL/SLS are all very low frequency). If not SLS, then like Athena, used to retain a potential capability long term.
However, NASA and AF have had interests in low cost, high frequency launches under the theory that below a certain threshold they could engage in more effective "usage of space".
In that ambition, where does "keeping alive" OA launch appeal to any govt (or commercial for that matter) interest? Don't get it?
Agree with woods170, that Antares might be one of the early victims of the current congested launch provider market, as perhaps Atlas V gets "cheap" enough in volume that it could have.