Half this thread is banned. Is everything ok?!
4 months for a mythic clear of a raid seems awfully short, especially if there's decent amount of bosses. It promotes split raids even further and makes lucky +ilvl drops even more important. Sure, not everyone should clear mythic, but setting cutoff point at ~300 guilds is going too far. Unless the raids are somehow much easier, which I doubt.
Besides, didn't they learn from the past? Making shorter raid tiers doesn't matter if the final one still last for a year or more. Better to stretch those a bit further, so the content gap isn't as bad.
When they made it so that each new tier completely obsoletes the previous one. People still ran Karazhan up until the day we could board the boats for Northrend. People didn't even go near Naxx after Ulduar was released.
What they need to do is have much more overlap in item levels between tiers (irrespective of whether or not they compress the item level spread within a tier).
Cheerful lack of self-preservation