Google doesn't start preloading 50 documents, it loads the top result or 2. Even viewing a hundred amp pages in a month is going to cost you less data than one 10kx10k unoptimized image that a publisher decides to load, which happens a lot. Note that AMP optimizes images too taking into account your device resolution, so it's most likely saving you bytes.
Everything else wastes my bandwidth and I don't care that it is only 100kb, my monthly, effective datacap is about 50MB and I always browser without images.
The "100kb or less" is only about 500 pages preloaded, not accounting for the size of the google webpage itself. I'd rather not have any preload at all.