A let's play is not transformative. You are literally experiencing the game as it was meant to be experienced. Similarly it's not transformative to put a song on in the background of a stream. (Video game music is copyrighted too ;)) For it to be transformative the game needs to be transformed into something else.
>it reproduces a small fraction of the whole of the work - only renders of the final work not the models behind it, none of the code, only the fraction of the artwork that is interacted with and even then only in one of many possible renderings of it
There are plenty of let's plays which play through a whole game. In terms of fair use you could for example illustrate what a boss looks like when talking about it by showing a screenshot of a game. Using your idea of what partial means you could watch a whole movie. It's not like by watching a movie you get access to the raw audio and video clips, or the projet files used to edit it. You don't get the 3D models they used to make CGI.
>it doesn't compete with the original work in the market place
For some linear story based games you could make the point that it does compte because you can experience the whole story without needing to purchase the original.