Tom:
His thoughts in G.A. echoed the principles he had laid out in 1914, so I'm not sure if citing post WWI economic issues works.
Like you, perhaps Mackenzie was blessed with the opportunity to work with ground that was ideally suited. I'd imagine its easier to let a few additional bunkers slip in on sites that already contained blowouts and exposed sandy areas, let alone being sand-based. In any case, from all of his writings it certainly seems he viewed constructed bunkers almost as a means of last resort, in situations where other natural features didn't exist to serve the same purpose.
I don't think this is a black and white issue. It is entirely possible that the timing of the depression lead him to give greater credence to the thoughts on economy that he had laid out earlier. But I think those initial thoughts were derived from his views on how the game should be played, not on how much money should be spent to keep up the playing field.
Sven