React content is fine if the person actually reacts. The problem is Twitch streamers have developed this grind culture where they're live all day every day to catch as many passerby-viewers as possible despite not having that many hours of stream content ready, so they pull up videos and let them play while they eat lunch or whatever, rather than just ending their streams once they've run out of things to talk about.
You can talk about the ethics of reuploading entire videos with reaction commentary all you want but it's ultimately content people want so it's not going away. I don't think reaction material really cannibalises views from the original all that much, since most of the reaction streamers' audiences likely wouldn't have been aware of the content until the streamers show it.
