This could prevent my next phone from being a Pixel.

But it doesn't make sense for them to do that. They can't just sell devices promoting them as "unlocked" then brick people's phones or locking them out so they can't access their data down the road.

Now, I agree that the long term future of GrapheneOS, if it has one, probably isn’t Google hardware.

Maybe, maybe not. 10th generation Pixels can be supported, so it'll be a while still. But GrapheneOS is in talks with an OEM and it's looking likely that they'll have official support for GrapheneOS for their devices soon enough.