GnusmasTHX wrote...
A) There's Shepard and his allies. Which includes; at least for me, a rapidly growing rachni population and an the most heavily armed ship in the galaxy, among other things.
Well, you've got Cerberus, a Rachni population that's had two years to grow, and a very *advanced* ship (I don't think the Normandy is really a warship though, more a kind of light starship with some combat capabilities). But no buy in from the Council, the Alliance, Turians, Asari or Salarians, which is effectively every combat ship in Citadel space.
Though I do think the Rachni will have an important part to play in some way in ME3.
It could literally take decades for this to happen, without a Plan C. Reapers don't want this. Even now we have miniaturized their main weapon. (And making things smaller is the truest sign of technological and scientific advancement, for reals.)
C) Even WITH the Relays and using the Citadel-trap (Perhaps the Citadel is a Relay control hub, turning specific Relays on or off to isolate groups to kill off, who knows.) Exterminating the galaxy was an extremely long and tedious process, as pointed out by Virgil, and even then, the Reapers weren't thorough.
Decades for what to happen? For them to fly back? If so I agree, it should take decades (at least) for the Reapers to fly back.
Which arguably would work to their advantage in one way, that if people barely believe they exist now, two years after Sovereign, they'll definitely have forgotten them in another few decades.
The Citadel is a relay control hub. Vigil explains that in ME1, that the Reapers used the Citadel to cut off other mass relays and isolated all the different planets of the Prothean empire by picking them off one by one. There's no suggestion that any of the existing races are aware of that capability (or indeed any suggestion in ME2 that there's any knowledge that the Citadel is a mass relay at all).
If all else fails then presumably the Reapers could *eventually* make it back to the Milky Way. But if they can do it in a reasonably short time span (i.e. a year or two) to incorporate into Shepard's story then I think it seriously devalues events in ME1&2. If it takes a significant period of time, i.e. decades, then it can't fall into Shepard's story arc.
Given the amount of comment and hinting in ME2 about dark energy, and the importance of Element Zero, I can see something along the lines of the Reapers constructing a "one shot" mass relay (a "no return" that requires victory in Citadel space), or I suppose some kind of stockpiling of power to allow a super-speed FTL return to the galaxy.