They do experiment and try new things, have done since Heavensward with Diadem and Deep Dungeon, they just don't really mess with the core of MSQ, dungeons, trials, and raids.
Given how well some of their experiments have gone over, it's probably for the best that their experiments with new things are mostly side content.
Can you imagine if instead of being side content, they saved it up and made it the focus of Stormblood. Every main area was like the original Diadem with all the dungeons being deep dungeons.