It's extremely simple: Raiding is losing popularity because people don't get satisfaction when killing a boss.
You kill a boss in LFR
Then Normal
Then Heroic
Then Mythic
Is this fun??? NO!
Can you skip any of these difficulties? Some -- but then it makes no sense why you would (for competitive & social reasons).
Back in BC (yes, the best expansion by a mile) you could spend several weeks on a single boss... and when he died --- you celebrated!!! Pictures, screaming, celebration, gloating.......woots! Did you want to re-kill him in the next difficulty??? OF COURSE NOT!!!
1 Difficulty....that gets slowly nerfed. Such an easy solution! Everybody can play...and at their own pace.
PS. Catering to casuals should not mean letting casuals clear the entire raid zone on day 1.......