I think the OP was referring more to educational material on channels like Discovery and History giving way to fiction based shows. Not that fiction based shows are "getting dumber". That's a different topic.
This is true, but he also suggested that 'network decay' was something that was happening on every network.
Since 'network decay' can mean networks drifting away from their stated niche (like American Movie Classics going from the channel that airs old movies to the channel behind Mad Men, Breaking Bad and the Walking Dead) I'd submit it's not that bad a thing.
Intelligent documentaries still get made, and with streaming services you can watch them at your leisure. We have more flexibility in how we consume TV content then we ever have before. Even if you prefer say older shows, I don't think anyone can fault the evolution of the distribution system that much, even if this means among other things a shift away from networks.