Name change should be up for debate, I think if its going to be a new nation then it should it be a new name as well in my opinion.
The Birth rates will reduce gradually over time as development occurs, but right now there is very little development in Afghanistan especially in Pashtun areas because war frequently happens there, and so long as that continues then the birth rates will remain where families will have 5-8 children, and even if development occurs, those birth rates will drop over time.
If Pakistan doesn't have a problem with us leading the nation since we will become a majority in the near future, then I personally don't mind the merge. However I do have a problem if Sharia is not going to be our lead role in life... If some provinces don't want to have Sharia then fine so be it. I say it should be up to the provinces, I know without a doubt the souther Pashtun areas of Afghanistan (Helmand, Kandhar, Nangarhar, Paktia etc.) the people want Sharia, and this is something that has to be respected and is something we will not compromise upon, we are not going to have secularism around these areas, if Punjab, Sindh, Karachi and the rest want secularism, that is their business. Let's see what will happen in the near future InshAllah.
I like that map, but name is up for debate, the same with flag. I simply believe, new country = new name + new flag, you are free to disagree, but that's just how I feel about it.