As some one has said elsewhere that they have shot with even 10 Mp camera & taken large prints with no issues.
How ever lot of things have changed & we do have good choices thanks to technology
I myself use D 500 & D 850 with 500 PF(i have two of them).
While i do both wild life & bird photography i do comparitively more birding (mostly hand holding) & the most challenging is shooting small birds like sunbirds in bad light & invariably the ISO goes up to 4000 or more
(I am sharing an image of a purple sunbird shot with D 850 + 500 PF yesterday with 1/400 & 4000 ISO & denoised with Topaz denoise.I never got such quality with D 500 in similar situations)
While i have taken lot of very good shots with D 500 specially action shots .i did find that it is D 850 images when cropped (with high iso) which comes a topper compared to D 500 (after using noise reduction software). I expect the same with A1 & R3 (since the two camears being compared are A1 & R3 based on their MP )
R3 is priced just 500 $ less than A1 & why would any one select R3 when A1 can do every thing that R3 can do & more (discounting the eye tracking which is supposed to be reintroduced with R3)
(I also am in the photo-craft business where my images get printed in variety of surfaces including paper,metal,glass,canvas,fabric,enamel, laminates, PVC,Magnets & even floor mats)
Hence high MP with similar costs is a no brainer for me

I do agree that mine is a worst case sceanrio .How ever this high MP (keeping similar costs & other featurs ) choise does make one's life easier
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