These really long periods without raiding content can be annoying, so in an attempt to decide whether or not to get Legion, I made some graphical representations of the density of raiding content. I neglected to include world bosses and Naxxramas 2.0, and I could probably have pruned away things like the Obsidian Sanctum. Aaaand I based things of when the bosses were killed, as opposed to when they were technically released.
It's worth noting that the midpoint in terms of bosses introduced is Ulduar. That is to say there were roughly as many bosses made before Ulduar as after.
The gaps on the right side of the graph are rather striking. But not entirely unprecedented when you consider that the gap between BT/Hyjal was filled by Zul'Aman, which wasn't quite suitable for most 25-man guilds. I'm not sure what blizzard can do to avoid this issue, but consistently having gaps of over a year between raiding content is going to make it more common for people to stop playing a year before each expansion.
The code can be found here.
Credits for compiling the dates go do Yarini, whose reddit post I stole shamelessly from.