At the end of the day Bill it comes down to what the buyer's purchasing power is and type of birding , if its for home use in the garden or at a zoo go for it, the zoom will definitely come into play. but if they are anything like myself then range is everything as in a lot of my birding is done walking on the cliffs of West Cornwall, fieldcraft does not really come into play very often especially with flying birds which can pop up from below a cliff an drop off just as fast, plus I enjoy trying for the over shooting rarities which from my experience are nearly always seen at long range.
I am happy hand holding the 500mm and i'm in my 60s I have used a 600mm, I could handhold it, but given the choice then no way would I get one, the 500mm is spot on for
me.