Unless the next best thing makes you think you are already achieving the "perfect solution" for what you think you care about, but in truth does not.
I'm not a mobile phone security expert but my feeling is that in the case of GrapheneOS - which target is probably high-profile people at risk of state actors et similia attacks - a zero-day in the closed source firmware from Qualcomm will probably screw you anyway.
I understand that you are anyway reducing the attack surface (now they need to target the modem firmware specifically), I understand the concept of security in depth and I also understand that by using GrapheneOS you are already placing mitigations for many other known and unknown attack vectors. But still...
Unless the next best thing makes you think you are already achieving the "perfect solution" for what you think you care about, but in truth does not.
I'm not a mobile phone security expert but my feeling is that in the case of GrapheneOS - which target is probably high-profile people at risk of state actors et similia attacks - a zero-day in the closed source firmware from Qualcomm will probably screw you anyway.
I understand that you are anyway reducing the attack surface (now they need to target the modem firmware specifically), I understand the concept of security in depth and I also understand that by using GrapheneOS you are already placing mitigations for many other known and unknown attack vectors. But still...