A. Why do you think it should be pulverised. Cratons are, by definition, stable regions. They are subject to minimal tectonic activity in terms of magnitude and duration.
B. Granite is a rock typically consisting of quartz, feldspars and ferro-magnesian minerals. Quartzite is almost pure quartz. Granite is an igneous or metamorphic rock. Quartzite is sedimentary.
C. The only way you can convert a granite to quartzite is to uplift the granite, weather it, separate out the quartz grains, carry them to the ocean, deposit them, rework them multiple times to round off the grains, subside the bed, lithify it, then subject it to high temperature. Quartzites are found in many parts of the geological column.
D. Rhyolite has the same chemical composition as some granites, but is fine grained, having been erupted at the surface, or emplaced at shallow depth, so that it cools rapidly. Many rhyolites have been formed from the remelting of granite plutons.
E. Many granites through mupltiple periods of activation.
Thus the granites we find have undergone a wide variety of events. Some have been eroded, their most resistant minerals redeposited and converted to quartzite. Others have been remelted and reinjected into the surrounding rock, or erupted at the surface as rhyolite. Some have undergone other changes induced by heat and pressure changes, leading to modified chemistry, mineralogy and structure.
In short the changes you say we should see, we do see.
I'll let you respond to that before we move on to the next one.