I agree with you that there are plenty of times when a tripod would limit a photo opportunity and a person has to go handheld or rely on a monopod. I’m thinking primarily slogging through the sage fields, woods, and river near Gros Ventre when looking for moose in the Tetons or following an animal along a road or field as it moves along. But, respectfully, using a high resolution camera in those situations in lower light, mornings and evenings, or just shade, means shooting at higher shutter speeds even with excellent technique, and that translates into higher ISOs, more noise, lost dynamic range, and lost detail. So, while I agree with you generally, and in those cases rely on my 180-600 vice the 800 or 600, I do whatever I can, and recommend, bringing a light carbon fiber tripod or monopod along with me whenever possible, and if absolutely necessary relying on a solid tree.
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@Steve, with today‘s high resolution cameras, the hand held reciprocal rule (shooting at a shutter speed equal to the reciprocal of the focal length of the lens) is out the window and more nearly twice the reciprocal rule is required for an acceptably sharp photo. So with a 600 mm lens handheld, the recommended shutter speed, especially when chasing a subject and either excited or out of breath or both, with proper technique is 1/1200 sec or more. With an 800 mm lens the minimum recommended shutter speed is 1/1600 sec. On a tripod or monopod I can bring that shutter speed down to half, or less, saving at least two stops of ISO and more if I have time to shoot a series of shots at ever decreasing shutter speeds and relying on the law of averages.
So, while I’m not disagreeing with you, I’m just emphasizing going handheld with the long prime should be a last ditch solution and not a first choice.