What were the limits of FTL travel in ME universe again? 12-30 lightyears/day? This would mean that if the fuel problem (eezo) was solved, one could cross the entire galaxy in 27-10 years. Not a problem for certain long-lived races. Even then, Arcturus cluster/Exodus cluster is only around 40 lightyears from Earth. Not exactly a problem for humans to reach their nearest colonies.
The theme of civilizations of past cycles always relying on ancient technology to drag themselves from their home systems, rather than advancing on their own terms, and their civilizations consequently being reduced to stagnation and henceforth ultimate destruction, is almost Dune-like.
That said, the nature and origin of the Reapers plus their way of defeat could've/should've been handled better. BioWare could've taken a lot of cues from the Butlerian Jihad (Battle of Corrin) books (as mediocre as they were) and Heechee chronicles.