There tends to be a much longer delay after the last tier relative to content patches. When the other tiers tend to be out for a couple months but the last tier is often about a year long, this is keeping people interested longer. People who might otherwise brick-wall and just quit until the next expac instead have something to work towards and keep subscriptions active. Blizzard has numbers they will probably never show us, but once someone discontinues a sub, there is a percentage they just never get back. It happened big time to my guild with MoP. They would just assume keep people busy with something.
"he doens't need healing, he doesn't need healing, he doesn't nee-WHAOSHIT!wtf was that man!". Please stop leaning on TDR. -Teng