Brunswick was the closest and firmest ally that Prussia had and the strongest permanent ally. The two sisters married the heir AND the spare of Frederick William I, and their brother Ferdinand was arguably the most important general of the Seven Years War. How much more important do you want?
we were talking about a 'true' power, like France, Austria, England, Sweden or Russia(even denmark), those are thing more to consider...
I think Old Fritz relucant to have childern were more his own chilhood traumas: All the physical, verbal and emotional abuse who his father make him endure to become the 'soldier-king' (with the dead of his best friend maybe lover Hans Hermann von Katte being the most traumatic event), and historician send to agree that even old fritz hate his wife since the begin:
"Frederick William considered marrying Frederick to Elisabeth of Mecklenburg-Schwerin, the niece of Empress Anna of Russia, but this plan was ardently opposed by Prince Eugene of Savoy. Frederick himself proposed marrying
Maria Theresa of Austria in return for renouncing the succession. Instead, Eugene persuaded Frederick William, through Seckendorff, that the crown prince marry Elisabeth Christine of Brunswick-Bevern, a Protestant relative of the Austrian Habsburgs. Although Frederick wrote to his sister that, "There can be neither love nor friendship between us," and he considered suicide, he went along with the wedding on 12 June 1733."
(A POD when old frirtz married Maria theresa can be a great timeline)
Thus being chidless can be atribute to his own traumatic childhood, If he would have a better childhood that would not have been a problem