It would probably be worse for us than the Reapers arrival though. Reapers, at least, harvest us and are done with it after. Protheans, however, used other races as slaves, so, should they have defeated Reapers with their hidden fleet in our cycle, our fate would not be very pleasant. Not to mention that, according to Catalyst, they would eventually create a race of machines that would wipe them and us out. So, maybe, Reapers are a less evil?
Which, again, makes it illogical that Reapers ignored this planet in their "reaping". Since they wiped out Inusannon there just a cycle ago, it would be logical to check it to see if no one used these ruins as a base. And to bomb them once again to make sure of it.