Well, there is a huge ilvl gulf that they can insert gear more gear into. Mythic raiders are at like 730-735 and Legion STARTS at ilvl800 (this is what the heirloom trinkets scale up to at 110) with normal/heroic dungeons probably being 815/830. It's triple the ilvl gap that existed between Mythic SoO and the WoD start point. First raid tier will probably be 845-855 for normal mode and 860-870 for heroic. That's about four times the stats of current Mythic gear which means they'll have to do a stat squish for Legion or hit the boss HP cap again in the second raid tier. So it makes sense that they'll roll out the squish with some content to both bridge the ilvl gap and make sure the squish is actually working.