Why do I think this? There's a number of contributing factors
1. As I've mentioned before, the massive nerf to tank threat gen, making it so that every week feels like skittish week.
2. Massive nerfs to the aoe capabilities of most specs also making them more tedious and difficult.
And the biggest reason of all...
3. Massive nerf to titanforging chances, making repeat runs of high level keys not at all worth it because heroic raids are easier than +15s, drop WAY more gear, and drop better gear. Since farming them will no longer be worthwhile outside of the first few lockouts of a new tier, almost everybody will simply do one per week for the chest and that's it.
Edit: Since people can't read, I'm going to clarify this here. The point I'm making about the thread isn't about how much you or I personally enjoy M+, I'm sure a lot of people on here love the changes, but keep in mind the people who post here tend to be far better at the game than 99% of people. The thread is about overall participation within the playerbase dropping off. Ghostcrawler said it himself in regards to the Cataclysm heroics being hard then subsequently nerfed. When content is too hard for people, most people don't "git gud", they simply quit the game or don't do the difficult content.