User blog comment:Legionwrex/Did the Leviathan DLC really add a new twist to the endings?/@comment-5287758-20120901035748/@comment-4721065-20120901062030

Leviathans were capable of hiding from the Reapers for so long simply because the Reapers didn't know what they were like, where they can live, etc. All the Leviathans had to do is to firstly erase all original records than hide from the cyces at the ocean depths on remote planets - something that is almost never explored.

On the other hand, you know all of that when you ascend to Shepbringer. As such, you simply send 300 Reapers to every water world (since there were at least 38,000 cycles by your own estimate, there should be enough of them for that, even after the war casualties) and begin the purge. If the Leviathans managed to to get decimated when there were no Reapers around and had to hide from the very first cycles when there couldn't be more then a couple of dozen Reapers, there is no way they would survive against that for any more then a month at the greatest.