You seem concerned about two issues: fair use and copyright term. Fair use is complicated and open to a lot of disagreement. Copyright term is more straightforward. I'll consider both in light of your examples.
Fair use is complicated because there's no hard and fast rule, just a set of factors that a court would consider and a few important court decisions to compare your facts to. Let's Plays are further complicated because some are probably fair use, but others probably aren't. An entertaining live commentary of part of a game would probably be fair use because it's like an excerpt included in a scholarly critique or literary review that is traditionally considered fair use. However, where the entire story of the game is included with all the dialogue and cutscenes, basically eliminating the need for someone to actually play the game, it starts to look more like an unlicensed copy of a movie, which isn't considered fair use.
Copyright term has a simple rule, it is finite. Alice's Adventures in Wonderland was published in 1865. When Disney released its animated film in 1951, 86 years later and 53 years after Lewis Carroll's death, the copyright term was over and Disney could make its movie without paying his estate anything. Today, the term in the U.S. is usually the life of the author plus 70 years, which wouldn't increase the term for Alice by very much. Also, Disney isn't special. The same freedom is extended to all of us. Any of us can make an Alice in Wonderland movie. We can use everything that came from Lewis Carroll in 1865, we just can't take what Disney added in 1951.
Disney has lobbied for increased copyright durations. In fact, by what you are saying (life, remaining I assume, plus 70 years), Disney couldn’t do anything with Alice until 1968 which would be 2 years after his own death.
I’m all for company’s right to protect copyright under the terms in which copyright was originally extended, but Disney is a big fan of moving the goal posts further out.
Fair use is complicated because there's no hard and fast rule, just a set of factors that a court would consider and a few important court decisions to compare your facts to. Let's Plays are further complicated because some are probably fair use, but others probably aren't. An entertaining live commentary of part of a game would probably be fair use because it's like an excerpt included in a scholarly critique or literary review that is traditionally considered fair use. However, where the entire story of the game is included with all the dialogue and cutscenes, basically eliminating the need for someone to actually play the game, it starts to look more like an unlicensed copy of a movie, which isn't considered fair use.
Copyright term has a simple rule, it is finite. Alice's Adventures in Wonderland was published in 1865. When Disney released its animated film in 1951, 86 years later and 53 years after Lewis Carroll's death, the copyright term was over and Disney could make its movie without paying his estate anything. Today, the term in the U.S. is usually the life of the author plus 70 years, which wouldn't increase the term for Alice by very much. Also, Disney isn't special. The same freedom is extended to all of us. Any of us can make an Alice in Wonderland movie. We can use everything that came from Lewis Carroll in 1865, we just can't take what Disney added in 1951.