*sigh*
I always wonder why these discussions always slavishly regard the figure of 50,000 years as if it is a stopwatch that begins counting down again when the Reapers arrive. And then they also assume that ANY civilization that isn't already spacefaring is safe from the Harvest.
Assuming both of these things simultaneously is LUDICROUS.
If a species just on the brink of spaceflight were to be skipped during one Harvest and then given an additional 50,000 years to develop without intervention? Do you think that the Reapers would just sit back and say, "Boy, this species is starting to be able to be a real threat to us. Too bad there's still another 47,000 on the clock. Oh well."
Let's be clear. Any civilization that has at the very least invented writing (and I would posit agriculture or metalsmithing as better benchmarks), will be likely to pose a grave threat to the Reapers by the time another 50,000 years goes by. The "Yahg are safe" always struck me as insane logic on the part of everyone.
Now let's take a look at what the evidence presented in the story implies. First off, the Protheans on Ilos disrupted the Reapers regular plan by altering the Keepers. It is heavily implied that the Rachni were originally being recruited to be footsoldiers for the Reapers way back when we were talking to the last queen down on Noveria (the oily notes souring the songs). This suggests that this Harvest is actually at least 1000 years LATE in happening.
So why DID the Reapers create the Mass Relays? Sovereign's claims make good sense. The easy access to a method of interstellar transportation that surpasses normal FTL drives is something that would stunt any research programs. Why work to figure out how to do something that is readily available? The Relay network eventually directs civilizations to the Citadel where great technology is just sitting, waiting to be used without ever understanding HOW it works. Again fostering complacency. So basically, the Relays work as roadblocks to technological development by giving easy access to advanced technology.
An easy analogy is this: How many of you have a smartphone? Now, how many of you can create an app to run on it? The answer from the VAST majority of you (even here on BSN) would be "Why would I put all that time and energy to write an app, when I can just go to the web and download one (or 50) that somebody else already made?"
My own theory (not backed up with concrete evidence) is that BOTH of the assumptions I pointed out at the start of this post are incorrect. I believe that each cycle is not a set period of time, AND that any civilization that has advanced to the point of writing is subject to the Harvest. I further posit (ZERO facts in evidence) that each Harvest is triggered by news reports via the Citadel (or the holdover such as Sovereign) of successful AI creations. If it were me trying to maximize efficiency in the Harvest/dormancy cycle, that's how I would set it up.