You're the only person who has gone beyond individuals to consider if whole units were affected - and that really was the essence of my point. Of course it is no suprise that many individual soldiers were war weary long before 1815, whether it was Picton, Ney or sergeant Bloggs. Just as many novices (and some experienced soldiers) were scared witless on the battlefield. But until I came across the passage in Alexander's book, I'd not seen any contemporary evidence that veteran units, strong in esprit de corps, regimental pride and the experience of victory, might grow stale and become less good with further exposure to battle. And I am still doubtful whether or not this applied to our period. This may partly reflect the fact that I read mostly about Wellington's campaigns, for I can't detect much sign of staleness in the performance of the Light Division in the closing campaigns of the war. Your suggestion about the turnover of men in the unit might well provide a clue to the answer - though it is hard to see how one can test it. Napoleon's Imperial Guard would be equally hard to assess given the pattern of bringing men in to the Guard for a time and then sending them out (with a higher rank) into the line as an example; and also the heavy losses requiring extensive reconstruction in the last campaigns of the war.
Can anyone suggest a unit - not an individual - which went from being an elite veteran unit to a less good unit without suffering very heavy losses?
Finally, as a counter-balance to all the quotes about war weariness/combat fatigue/shell shock and the like, I wonder if anyone has evidence of the composition of volunteers for the forlorn hope and storming parties at Ciudad Rodrigo and Badajoz in 1812? I would hazard the guess that they were NOT largely composed of Johnny Newcomes keen to cut a figure in the regiment, but that there were a good few veterans among them (including some who voluntarily took part in both).