The additions you speak of are over. Zero people were gained by the occupation of Bakhmut, it was evacuated, ditto for many other towns and cities captured.
The people will come back. Anyway, Russia won't take all of Ukraine, just what they consider to be important.
Donbas' share in Ukraine's economy was really great though. More than Kiev's share. The next big ones are Kharkiv and Odessa. That entire region is the industrial and farming belt.