Personally I think that the 400mm prime would be too limiting for the normal home user. Yes the 400 is an excellent lens. And if I was a pro sports TOG, or birder, it would be the most valuable lens in my kit.
But as a hobbiest, one must weigh up the pro's and con's. A bit over $1,000 for the 150-500mm verses $9,000 - $11,000 for the 400mm (Fleabay)
Is it going on a top of the pro range Canon/Nikon, or mid range body. Is the user needing high quality images to make a living. Or good quality images to show friends and family.
The 150-500mm is a good lens, and it take some practice to get it to take good sharp images. (maybe not as sharp as the 400mm) And if your not taking 8fps with AF-C or 3000 images a game to sell to some sports magazine. It would be a good lens.
Here is an example. A Sea Eagle doing its in flight business.
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