The only thing worse than no security is a false sense of security. It's better for someone to know they're using an untrustworthy device than to find out the hard way.

If other phones can't provide an acceptable baseline level of security, the GrapheneOS devs are correct to not support them. Yes, it sucks that the only phones supported are Google Pixels. They have a partnership with Motorola to create a non-Google alternative.

If you still don't like it, fork it.