I really really disagree with this one. Blizzard knew very well what they went for - they literally made Sylv Warchief 1st hour of Legion and teased Dreadlords serving Death legit first raid with Ilgynoth. They released chronicles with cosmology chart and told us we're going to explore it. You legit had Sylvanas going buddy buddy with Helya patch 7.0 and so on and so forth.
So yes, it's not some "shit we don't know what to do, quickly make up some shit" - this all was planned exactly like this. They could have made a few tweaks as they went, but yes they knew full well that in a few years from there we go to Shadowlands with Sylv going rogue and big bad Death-deity messing around.