Yes and no. For sure people leaving is due to multiple factors. But one of them is the fact that in the last expansion the whole focus of the game was to keep people online via mandatory chores, that weren't fun but you had to do to stay on par.
It's fun that a 3/4 months cycle is fine for you - m+ is the least timegated content wow has, you can literally spam them as much as you like and pugging is very easy (in terms of accessibility, not success). I would be fine with a 3/4 months cycle of atvthe emd of it given enough commitement fron players you could get to ilvl cap whatver content you did, even world quests.
And yes. Content is really umbalanced in terms of rewards. World content is unrewarding, casual content is neglected, m+ give much better rewards for easier content compared to raids (both quality and quantity). Reason why i'm in favour of a ilvl cap at 15s/hc woth world content catching up to that later during the tier (maybe when the .5 hits).
That would make also obsolete stupid catchup systems that need to be implemented evey patch because people have no way to get there without them.
My wet dream would be to have mythic raids work like the mage tower. Always scale you down, and it's always a challenge. If people can't farm them the expansion after, is not a problem - make those reward count.