It's not an open source phone. It's closed source hardware with closed source firmware. The closed source userspace drivers/services could be rewritten but the hardware and firmware isn't in the same situation.
Hardware and firmware level protections can't be added by software.
Software features based on hardware security features also can't simply be added without those. Translating code to add a software equivalent to MTE could theoretically be done but would make the code require several times more CPU cycles and several times more memory. You would need far higher performance hardware and a bigger battery for that to be practical. MTE is what GrapheneOS uses rather than that.
Depends what you mean by open source phone, do you mean open hardware? If not, you cannot retroactively add MTE or a secure enclave. Even if it was open hardware, it is kinda hard to do small production runs.
Sure, but then you have to run a different OS. That's completely fine of course, between Graphene, Sailfish, etc. there is a lot of interesting stuff happening.
Yeah, but I thing the best security is very important to the GrapheneOS developers, so I don't think they will ever compromise on MTE (except pre-Pixel 8) and a secure enclave. Of course, someone could fork GrapheneOS to support phones that don't fulfill the requirement. Also, I think fighting for the user is also true for e.g. LineageOS. As far as I know they are also a non-profit.
It is. But you can get almost all of the benefits on hardware without that. Someone has to port it, though - Graphene won't. Yes, it's also true for LineageOS and you can use that too.
No, you can't obtain almost all of the benefits of GrapheneOS on other hardware. Many of the benefits depend on having current privacy and security patches along with hardware-based security features.
