Pure speculation on my part: c-suite wanted the old regions written out with a view towards continuing to reinvent Dragon Age as a YA or romantasy setting going forward. Chasing the ever cryptozoological modern audience, tasteless and profitable, "unburdened by what has been" so to speak.
I don't know, I don't buy it. Isn't Tevinter also technically an old region? I think it's the decision of the writers, who were too high on their own farts. When you have no respect for the original work, it makes you do crazy things.
For example, in Warcraft 3, a big thing is made out of the foundation of Theramore - the Alliance of the old world fell, over the course of the Frozen Throne every part of it was either dismantled or corrupted - now, on its own, it's not the example of this, because the creatives behind it are the same. But in World of Warcraft, where we get fresh new geniuses behind the wheel, we find out that no - the Alliance of the old world is just fine, Stormwind magically came back from the brink, and its populace (which was either genocided, or turned into refugees by the orcish invasion) is thriving, it's the new bastion of humanity. No, it's Theramore that's fucked - in one of the expansion, it gets nuked, just so the writers can say "This time, the war is for REAL, this is an unforgivable atrocity, this bad guy has harmed humanity in a way no WoW villain has before".
Or even better example - the big mastermind behind everything in Warcraft 3 was Lich King Ner'zhul, a villain from the previous entry, every event in the game went according to his plan. And the game ends with his complete victory. Come WoW, they write him out of the story in a book, and he doesn't even appear in the Wrath of the Lich King. That's bad, but nowhere near as bad as what's to come - years later, with the new creatives at the helm, we find out that Ner'zhul was always just a powerless puppet, and the real mastermind was
their newly invented character - the Jailer.
This is actually EXACTLY what Veilguard does, as they introduce superpowered jews who have actually been controlling the series' most complex and beloved antagonist, Loghain. As well as everything and everyone else.