I think subs initially skyrocketed just prior to and after Legion launched.
I think all of those "come back to check it out" subs left within 3 months and that more regular players started leaving within 6 months.
The legendary weapon/artifact system is dog shit and made for a very uninviting expansion. Which is a shame because the lore, story, and quests are AMAZING.
Just imagine what Legion could have been if there was no legendary weapon/artifact system and no Diablo-styled RNG legendary items. THAT is an expansion I'd play nonstop.