It's fine saying go for a 600mm max range lens, or add a teleconverter to a 400mm, but if you end up missing shots with it (not locking on to focus quickly enough, or the aperture is too small to give you the shutter speed you need for a sharp image of a moving object without going too 'noisy' with your ISO), then it's probably no more use than a faster and more accurately focussing 400mm max range lens and a bit of a crop.
Unfortunately, all round quality and performance doesn't come cheap and you can't get a Rolls Royce for Mini money. So for many of us something has to give, be that AF performance, zoom range, aperture stops and/or weight and size.
Could it be that a lot of the 'lightly used' 150-600mm zoom lenses you see for sale have been bought by people who subsequently realised they're a pretty big lump to carry around and use hand-held, and found they weren't using them as much as they thought they would because of that?