Actually player power will greatly increase in the week before Antorus Mythic:
1. 2 ilvl 970 legendaries become 1000
2. Some players will RNG ilvl 1000 trinket off Argus the Unmaker
3. Ilvl 960 loot will be available in the weekly chest for completing a Mythic +15 in the Antorus week
4. New set bonuses
5. A lot of titanforges
I think its standard already that the first 3 bosses will die like flies, and most likely Argus the Unmaker is tuned for more ilvl than he drops(and he drops 970 i think).