Thank you for the mentions guys, I really appreciate it.
I will chime here too if you don't mind.
The first footprint that 90% of the PBNs do is full articles on homepage. I swear that 95% of the websites that have full articles on homepage are either PBNs or mommy blogs. By using this primarily filter you have majority of the PBNs already covered. And there are many more. As you said, sites blocking crawlers for example. Sites redirecting dead inner pages to homepage etc. Sites that have been dropped for a longer time period. If they can work with niche relevancy, then multiniche sites.
I mean, honestly, if Google really wanted to, most PBN users would be fucked hard already.
So apparently, they don't want to. Why? I honestly think that there is some "beyond-reasonable-doubt" policy. They can't manual-action-rape the site unless it's an absolutely blatant PBN. OR they have counted the ROI they would have if they dedicated more manpower for manual reviews or to adjusting the algorithms for finding PBNs and they decided to fuck that, it's not worth it. Either one or the other. Let's just hope it stays this way.
And about amazon sites not being profitable, I will have to disagree here. Even if you spend $5K to rank a site that makes $500, which, unless you are utterly retarded shouldn't happen even in the next few years, the ROI will be there in 10 months. If you are building long term stuff there is. And even then you can, relatively effortlessly, sell the website for 20x the monthly profit from the last 3 months, so for $10K give or take. I mean, yeh, throwing 5K to something that will make around 15K in its entire lifetime isn't amazing, but this is the borderline worst case scenario.
Still, the focus is being shifted to more of a authority site approach and content marketing lately.