One reason I think they are going to go with a new timeline universe is it free's them up in the way the Kelvinverse movies were free to ignore canon. Even if they go to the mirror universe your still in a universe that has to someday have the TOS episode happen and also later the DS9 stuff. Technically they would have that freedom in the Kelvinverse as well but that isn't a new location and I don't think they would want to connect it to much to those movies and I still wonder about the rights and how much stuff they can even use from them or not. Better to start a whole new universe and maybe in this universe Section 31 finds Khan and he dies at the end of a episode. Maybe Lorca is captain of the Enterprise and she has to end up working with this guy whose version in her universe tried to kill her. Stuff like that.
Jason
Jason