Absolutely, and when the implausible contrivances in every game have stopped them from ever being made?
This is a poorly constructed sentence, but I guess you're asking "when has that ever stopped them?" And you'd have a point.
What's implausible is the notion that the galactic civilization could find and expend every habitable cranny in the MW before they destroyed themselves (reapers or no) or evolved into something completely different. That's a double edged argument, I guess, since that's an argument in favor of staying in the MW - we're never gonna run out of places to go.
However, the idea that a concentrated attack on the MW's civilization base being enough to cripple that civilization and hinder any real attempts at expansion within the MW is more plausible as a catalyst (zing) for the flight to Andromeda. If that is in fact what precipitates the flight to Andromeda, because of course we don't know why we're going yet.
Ok, what is the argument?
The argument is that they lost the thread very quickly. ME2 starts on a profoundly stupid note, and doesn't really improve through the rest of that game or the next.
Focusing on a single PC, with a single conflict (or a different aspect of the previously depicted conflict), telling nicely self-contained stories, as they do in Dragon Age, is a much better approach.