The answer was simple. Just make an equivalent. They made it for every other job (Shoha II, Ikishoten, Foul...). Why not for DRK? Especially now that we have Shadowbringer. Like, they could have just taken advantage of old discarded animations and reworked them into new stuff.
But no, they decided to do this, which ended up gimping DRK's sustainability.
The rest about Foul and Xenoglossy, that's a problem with BLM itself and should be addressed, sure... but for what's worth, for this conversation, it's important to note that they did make skills for stuff similar to our single-target combo but for AoE. So it wouldn't be out of the realm of possibility.