The Reapers created the Citadel and the entire mass relay network, so they can unlock every single relay in the galaxy without fear, because they knew where they all led. Whereas the younger races who simply discovered the relays and the Citadel could only open relays once they knew where their partners were located, for fear of accidentally encountering another hostile species like the rachni. So the Reapers would be able to find every single planet hosting a technologically advanced species, even if they had yet to discover mass effect technology. I recall reading a description of an uncharted world in ME1 that was home to a civilization with a Bronze Age level of technology, but it had been devastated by an extinction event and all that was left were impact craters, which were proven to have been targeted at population centers.
If humans hadn't discovered the relay orbiting Pluto, I'm pretty sure the Reapers still would have harvested us, because even though we wouldn't yet have reached that tier, by that point we will have been on the cusp of achieving it, which means we would have had 50,000 years to advance and grow stronger, by which point we might have posed a very significant threat to the Reapers, which they obviously can't allow.
If humans hadn't discovered the relay orbiting Pluto, I'm pretty sure the Reapers still would have harvested us, because even though we wouldn't yet have reached that tier, by that point we will have been on the cusp of achieving it, which means we would have had 50,000 years to advance and grow stronger, by which point we might have posed a very significant threat to the Reapers, which they obviously can't allow.
