While they were not immediately affected by order 66 , if you Wreckers activated before it could be removed which shows that their “defects” just stalled its activation

I think the show could have easily ended S15 Ep19..everything was wrapped up, Jack became what he was supposed to be and the boys drove off but I read a theory that works (for me anyway) about s15 ep 20 ( Carry on). Yeah Sam and Dean were awesome hunters and survived multiple deaths and so on, when Chuck was writing the story, and if you they were his "favorite" characters, so he couldnt let them die. Once Jack took over and said he would he hands off, the first case the boys went to without the plot armor Chuck was providing, Dean dies to a simple piece of rebar on what should have been a routine vampire nest clearing.