North American presented the P-51 to the British, because they were asking North American to built the Curtiss P-40. So, why other aircraft companies would not be able to produce the P-47 under license? I considerated that an aircraft similar to the P-51 could have been developed (there were some if I'm not wrong), but since the thread specifies the P-47, I didn't suggested this.
Also, it wasn't like because 15,000 Mustangs were built, that 15,000 Mustangs were critically needed and consequentely 30,000 P-47s would be needed. It's good to have a big reserve by several reasons like: replace lost planes, maintein high operational % of squadrons by puting new planes in place of those that needed extensive repair and overhaul, replace worn-out planes, and also the fact the Commanders were not aware of how long the war would drag on, and this included the Japanese and perhaps even a Soviet treat.
So, there are many considerations for this situation, but I beat one thing: there would be no salvation for Germany without the P-51, perhaps only a delay in it's defeat.