yes, it always was from the moment it was introduced, and as of writing this comment only 7 guilds in the WORLD on alliance side have managed to kill mythic jailer, while the horde side is already full, extrapolating out gives horde a 95% (allowing a +/- of 2% for errors) population dominance over alliance at the high end.
as to the question posed in the title of the thread, no, it's not the hardest raid tier ever, it is however the hardest raid tier for the current player population to participate in, of which there is a staggeringly small number of players left who actively raid:
using wowprogress as the measuring stick here, and assuming that the rate of number of kills stays the same between now and the 3 month mark post mythic becoming available, it's possible to get a world top 2k rank for your guild by only killing 5/11 mythic bosses, that's less than half the raid done while achieving such a 'high' world rank, to get the same ranking at the height of the games popularity you needed to have cleared 90% of the bosses in a raid on the hardest difficulty within a month of that content being available to you.
at this point in time i wouldn't be surprised if WoW had less than 1 million players globally (not counting china), and of that total playerbase less than a third are actively raiding, and of that ~300k (assumed) less than 20% of that raiding playerbase actually engages with the hardest difficulty mode 'Mythic', which means that these nerfs that blizzard are pushing out are not because the content is too hard, it's too hard for the players that are left to actually engage with it and they are nerfing it to allow actual progress to be made by the utterly tiny population left that engages with it.