A billion credits in a day? OK keeps telling yourself that lol.
By the way I don't care what people do in game - no moral stance on this. But I'm not willfully blind.
It's not an exploit though. An exploit is taking advantage of UNINTENDED gameplay. This is clearly intended. YOu can say the skimmer spawn rate needs checking, or you can say the rewards need reducing, or at least scaling with the security response of the base, but htis is not an 'exploit' in its normal definition. You could say a development oversight.
There's a reason for it, and in that reason of course, is the potential solution.
You've probably seen yamiks vid, where he has this base where he can go and literally just keep firing missiles until all the missions complete (in fact he wastes his time logging in and out, you don't even need to do that any more, there is a never ending stream of skimmers, you only have to reverse 500m then forward 500m and attack the new spawns). The problem here is how much pay it is for how easy it is to complete.
Now, I personally, don't have any bases THAT easy near my home. When I do this, I use three different bases (so I can stack enough missions without going crazy logging in and out), of three varying security levels. One is a bit like Yamiks one in that it only has two small turrets that are taken out with a single missile, then I can shoot the spawns, but hte other two are both high security, and have power plants that need to be taken out and kept down (they self repair), otherwise the Anti Air defenses take you out quick. I don't consider that the way I'm doing these is exploity. I hope that when whatever nerf comes for the fish in the barrel method, it doesn't affect the high payouts for the strategic assault of high security skimmer bases.
Therefore, as hinted previously, perhaps the rewards need to scale with the security response, or rather, only bases with meaningful security are selected for these missions.