Most of Affliction warlock's strength in Mythic+ is due to the prevalence in power per
Éclats d'âme utility. Before, you would use a single
Éclats d'âme to cast a single
Affliction instable on one target. Not very impactful, except for focused single target damage. Your only real AoE choice was to spam
Graine de Corruption.
Now, with the addition of
Extase maléfique, you now have a strong nuke that hits every single target affected by your DoTs. To break down what a quick AoE pull could look like for an Affliction Warlock now, with no major cooldowns, you can throw a few
Agonies out, cast one
Graine de Corruption to apply
Corruption to everything, cast
Souillure infâme, then spam
Extase maléfique until you run out of shards. While this may not be the most ridiculous AoE output of any class (see Destruction Warlocks), it's essentially available for every single pull. In addition to this, Affliction is typically known to be one of the best scaling specs in the game - with endgame level secondary stats, it's no surprise that a spammable AoE nuke has nudged Affliction into the limelight.