In such discussions, I find it curious that the R&D efforts of the US and UK are somewhat neglected or liightly regarded. With an evident need to produce a long range air superiority fighter, I suspect, in the unlikely event that the P-51B and subsequent marks were never developed, something would have quickly evolved to take its place. I have to agree that the immediate absence of the P-51 would have caused a delay in the Reich's defeat of the order of months but not years. While I appreciate the rational basis for the economic argument that limited resources and funds would have hampered efforts to field an equivalent number of P-47's, I believe that the US could have filled tha gap without too much trouble in reallocation of resources. WHile an unproven assertion, I suspect the amount of excess in the US Military budget could have absorbed such a demand without significant impact on other manufacturing priorities.
I also find that arguments of the intractability of cross service use of aircraft types to be a bit overstated. While rare. I believe there were examples of the USN and USAAF adopting one another's designs when performance showed a type to be superior to the aircraft then in use by a particular service. Well known examples include of course the B-24, designed as an army heavy bomber and adopted by the Navy as the PB-4Y-1 long range land based maritime recon and ASW aircraft. The army adopted the Navy's SBD Dauntless as its A-24 Banshee. The USN even tested a P-51 with a notion that it might be adapted to Carrier use although when that happend is unknown to me. One might say, these exceptions 'prove the rule' but with equal validity suggest that, in extremis, such predjudices can be overcome. Longer-ranged, de-navalized USN type alternatives to USAAF fighters such as the F4U-1 seem to me to be a valid option to be considered in this thread, especially as escort fighters providing somewhat deeper mission penetration than the P-47D escorts might have provided. IMHO.
Of course this is an aside from the thread's basic question. In that regard, I expect that absence of the P-51 would have accelerated development and deployment of the P-47N, whose ETO need, after the Fall of '43, would have been very apparent but still come with the aforementioned delay during which time airfields closer to Germany's heartland may have become available, assuming Luftwaffe numbers did not impact the Normandy landings.