The problem is not GrapheneOS, but rather that phone manufacturers other than Google don't care. Now if there were millions of GrapheneOS users, it would start becoming interesting for other phone manufacturers to care.
My point being that I buy Pixel in order to give more weight to GrapheneOS, in the hope that other manufacturers will eventually realise that.
E.g. a new Pixel 9a is currently 369 Euro in The Netherlands and 367 Euro in Germany. The Pixel 10a will be released soon, but the 9a will run GrapheneOS just fine (same SoC except modem as the vanilla 9).