I think that's a worthwhile point to consider but it's only relevant if we move the goalposts from "GrapheneOS is only used by Android ROM enthusiasts" to "GrapheneOS is only supported by one small Android phone manufacturer".

To be frank, though, I don't see any of this line of reasoning as relevant; it's just appeals to greater authorities on either end. If AOSP is only for manufacturers there's really no reason for it to be open source in the first place. And then folks who care about actually improving security end-to-end outside of whatever's convenient to implement by those beholden to the quarterly profit metrics are up a creek.

Personally, if this whole GrapheneOS/Motorola thing doesn't improve the state of the ecosystem I'm going back to Apple or whatever other manufacturer makes it clear they take security seriously.