Perhaps traditional 2D artists should start giving credit to all the various models they pay to pose for them? They definitely should give a detailed credit list for those “things” which exist, whether man-made or nature made, which they use to create their works without paying for their use. 

I’m sure Leonardo did not give formal credit to the model who was the basis for the Mona Lisa, so Leonardo must have lacked some artistic ethics – according to your post.

I generally make, or heavily modify any foreground models I use in my scenes.  I usually modify my Poser textures and do post work on them.

There is an assumed consideration of courtesy in giving credit when using a model which someone donates for free use, but to place a moral judgment call on those who fail – often innocently, to include this courtesy is a bit overreaching.

Those who purchase an item for inclusion in their work have no obligation to give credit to the vendor; they give credit by purchasing the item.  In any other area of commercial activity the absurdity of giving credit for the creator/seller of products would be self-evident, so why should there be such a silly practice only in the world of 3D art?

If you use a model that was given to you, then give due credit – unless it was years ago, and you don’t remember where it came from.  If it was a model of a Ford Truck, then give credit to Ford Motor Company – I doubt the modeler did that when giving the Ford model away for free.