Sorry, I stand corrected: Despite owning a Pixel 9 Pro myself, I didn't realize Google's Android phones market share was so tiny (1.1%! according to https://www.appbrain.com/stats/top-manufacturers). In that case, of course, the number of Pixels bought because of GrapheneOS could be a significant fraction.
That’s 1.1% of ~1.1-1.2B Android phones sold per year. I dont’t know how long the average GrapheneOS user holds on to their device, but if 25% of them per year buy a new phone (rather than upcycling a used one or using their previous phone another year), that would amount to about 1% of Google’s annual sales at best.