To be honest, there’s so much data, websites and variables out there that penalizing on fixed rules would result in a lot of false positives. For example, websites belonging to the same company and all linking each other not necessarily for SEO but to let people know those sites are related/share ownership. Not to mention negative SEO of course.
I think google should stop penalizing sites altogether. If google thinks a link is abusing its guidelines, it should just ignore it.
Like oh this site suddenly has 10k spammy backlinks? Just ignore them instead of penalizing and de-indexing the site.
If google thinks a site is a farm (aka a pbn), then just ignore the obls instead of de-indexing the pbn and penalizing the sites linked.