They get away with everything. Their family got back together. Against all fucking odds, they've managed to fool the FBI, they've managed to outplay the cartels, they've managed to avoid getting killed for redneck girl's chicago kill, they have settled their scores with everyone, all of their enemies in the ozarks are dead, their family is united and stronger than ever, the PI is gone and now the father too, everything is going just perfect...
and then the private investigator shows back up again up. With actual proof this time. A lesser show would have gone for the shock ending of them being fucked out of left field, arrested at the end, going 'We would have gotten away with it if it wasn't for the middling private investigator!'
Instead, it's someone else who underestimated the Byrds and ends up being killed by Chekov's shotgun that made its appearance in the first episode, in the hands of the one it makes the most perfect sense would have used it to kill someone.
The bad guys got away with it. They got away with everything. Even that last murder, they will get away with. He's gonna get cremated in their furnace like Wendy's brother was, except this time they'll get rid of the evidence, they will clean the furnace just to be sure, then they will be on their way to Chicago to start their new life..
Shit at this point they might has well rob liquor stores on the way for the fun of it, because they'll get away with that too.