Can't agree with the statement that it's not a defect, but I agree that it was always there since they made the bike. It's a manufacturing defect. Failure really depends on how much and how hard the OP proposes to use the bike. I am pretty sure these were made by squashing the stays and then the stay and dropout were held together by a couple of spot welds. As long as the spot welds hold, these cracks aren't going to matter. I have had the misfortune to work on a large number of bikes made this way and I don't recall seeing one that failed at that location. But it's possible the owners would just immediately give up on them and not take them to a bike shop.