I rarely watch any of the big retro gaming channels anymore, but I don't think that this necessarily due to saturation or that there aren't enough topics remaining to be covered. I have noticed the same thing with channels dedicated to other stuff,such as movies, guitar, science, etc. I think that these large channels start losing their appeal due to the nature of being self employed and successful. With a regular job, you typically have a rigid schedule and less flexibility in what work you need to do. All of our jobs involve some tasks that we would prefer not to do and would probably avoid doing them given the choice. Nonetheless, our employers hold us accountable, so we get them done.
For the big YouTubers, that isn't necessarily the case. While I don't want to call them lazy, once a channel reaches a certain size, it's easier to get by without putting in the same work that made your channel popular in the first place. I've seen this with a number of channels that moved from scripted content to live streams. Scripted content takes a lot of work and can involve a lot of research and video editing. Live streaming doesn't require all of the video editing and it is often just the host answering viewer questions. Given the choice of doing scripted content vs streaming, I think that most would choose the latter. Once your channel reaches a certain size, you can also make money reading super chats, so it further incentivizes doing less scripted content.