My not at all a lawyer reading is that streaming video games is almost always a classic example of fair use and thus not copyright infringement in the first place. Fair use is evaluated as a balancing act of 4 factors, and streaming is very favorable under 3 of them:
It's transformative, adding something new to the original work and not superseding it at all (factor 1), 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 (factor 3), it doesn't compete with the original work in the market place (factor 4).
The only thing that really ways against it is that the original work was very artistic in nature (not something like a recording of JFK getting shot, mainly of interest for it's factual content... actual example from a court case).
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.
Uploading a clip of you silently watching a movie the whole way through with the camera pointed at the screen would obviously be copyright infringement, but a let's play of a game does not convey the experience of playing the game.
If the entire experience (or even a large portion) of a game can be had via passively watching a recording of somebody else playing it then the developers haven't really succeeded at making a 'game', they've made a long-ass movie with extra steps.
And yes, maybe if you can show that your game is practically a movie that asks you to press a button every 5 minutes then you should be allowed to copyright strike commentary-less lets plays of it - but where exactly do you draw the line?
The player playing is turning the game into something new. The name would be something like "a run"
Something like Minecraft is even turing complete. You could describe just about anything in your playing, including like writing a novel.
Also, the moment you've got a multiplayer game, the whole thing switches to how players interact with each other. A conversation in game doesn't become part of the owned product of the game maker
Let's run through that kind of analysis here. Factor 2 disfavors fair use, because the original work is clearly "creative"; what more needs to be said? Factor 4 is the marketplace factor, and clearly, people who are watching the game aren't going to be playing the game themselves, so it's having a depressive market effect--fair use disfavored. Factor 1 is again disfavoring of fair use, because you're reusing market value copyrighted content in a commercial context. It's not transformative because it's not commenting on the original work [1]. Factor 3 again disfavors fair use, because you're reproducing essentially the entire video game in the playthrough.
Conclusion: all 4 factors disfavor fair use, so how can it be fair? I personally disagree with this analysis, and I even think that it misapplies SCOTUS-level precedent (precedent at the circuit court level is even more incoherent), but this kind of analysis is more likely to be what I expect to see, especially because (as in Thomas's minority opinion) the monetary and commercial implications of the use tend to dominate the thinking.
[1] My understanding of practical US fair use analysis is that "transformative" is viewed far, far narrower by the courts than by lay people.