Isn't that more or less what they did?
In this new book? Nope. The SCAG mentioned the second city, Djerad Kethendi, as well as the Ash Lake and some settlements around it. And most importantly, Tymanther was still existing as a political entinty (ie. the country).
The new book is categorical. It literally says "Tymanther is no more" (in the dragonborn species entry). The dragonborn got reduced to just a single city (Djerad Thymar, that for some reason is now carved on the side of a mountain, when all previous lore said it was a pyramid-city near the aforementioned lake) and some villages. The Ash Lake and Djerad Kethendi aren't mentioned and not even shown in the map (they just disappeared). And worse, the dragonborn now live in stlarning Calimshan (and consider it their country, worthy of dying for it, when there is no previous lore of dragonborn ever living in that place, so antithetical to their mindset, because before this book was Disneyland for slavers).
So, no. Is nowhere near to what they did.
but it gained an ally with one major population center in the north.
The book never states Messemprar is an ally of Djerad Thymar (it can be surmised, but it's not stated, so shouldn't be taken as a fact). In fact, what the book does state is that Messemprar is in a desperate battle to avoid being wiped out by Gilgeam. So, Gilgeam is this super almighty Sauron that hasn't conquered all because... reasons.
And if we go by implications, well, all the implications of Unther being a hell on Earth, of the blood splattered on the streets of Unthalass because Gilgeam allows no dissent, and all the atrocities the army of Unther commit... well, let's say I wonder how the dragonborn of Djerad Thymar avoided total extermination.
Yeah, I did what someone suggested and went to read the book (or well, just the relevant sections, because I don't have the book. A friend kindly showed me screenshots of the Old Empires section). I still don't understand why people prefers this to what we had. Like, I know things should have a certain level of tension and a bad guy doing injustice to have adventures in, but there should be also a place that is worth fighting for. The dragonborn fans lost that place, and we have to be grateful they gave us a single city (changed from what it previously was) and some shoddy copy of the Sultana's dragonborn army in buggering Calimshan...
And yes, this may feel nitpicky... But I have admitted it since the start. I don't like what they did.