And when you remove QBs from the calculation, this phenomenon becomes even more pronounced, because QBs are such high value and tend to be more reliable performers.

This is where you go wrong imo. The point of tanking is to get a QB, because having an elite QB immediately turns a team into a Super Bowl contender. And you can only get an elite QB through the draft. It would be wrong to tank for any other position, because no other position will affect a teams success the way a QB will.