About the transformative aspect of let's plays, how would that work between different genres?
I can totally see a Firewatch let's play working as an audio file you listen to while you play it yourself. The person recording it just needs to be a bit more specific about what they're doing now, and where they're headed to.
But what about something like this:
https://www.youtube.com/watch?v=3I59It79o3g
https://www.youtube.com/watch?v=Gdb85PoRgvU
How would that work as anything other than a video?
In movies it's much more black and white, there's no movie you can't have a "let's watch" through audio, but for some games it's just not possible to give the same experience as actually watching their footage.
I'd say it also applies to speedruns, they're doing things the devs never even thought were possible, there's a unique element added to the game, that even if there was no commentary at all, it just wouldn't be the same as playing through the game yourself.
I'm not saying those videos are at risk or anything like that, just really interested in the particularities of videogames when it comes to defining transformative works and fair use. You can make a very good case for a Firewatch playthrough with no commentary being nothing more than a tool for people to experience it without paying, but you can't say the same for a Dark Souls speedrun, or a DMC4 playthrough made by a high level player, and convince anyone who plays games at all.