The point of GrapheneOS isn't improving a generic device's security, it's about setting an example for a highly private and secure OS. It's a FOSS project, so nothing stops a committed individual or community from using other device targets, but the main project chooses specifically to use their smaller resources to pursue excellence rather than mediocrity.
I've had people tell me that nobody should use anything but GrapheneOS and stop supporting alternatives to throw all support into that because the others are "less secure", and now that GrapheneOS isn't for everyone and anyone -- the majority of people -- without a specific narrow selection of hardware should get lost.
We need the people who buy $100 phones to have the ability to put a better OS on them than the burning mudslide that comes with them, is all I'm saying.