A 'secure OS' which comes with a built-in VPN doesn't sit well with me (CIA says, pls send all ur traffic over our 'uber secure' tunnel kthx kek), and it seems Calyx isn't as technically adept as Graphene when it comes to kernel & library level exploit mitigation techniques.
The history of GrapheneOS is quite funny btw, when it was called 'CopperheadOS' the actual dev mic-dropped and deleted the signing keys stating that the business guy was compromised. Remember, the business guy was making people physically send their phones to HQ so the OS image could be 'securely installed'.
-Calyxos is less secure but more usable day to day, since most apps work. -GrapheneOS is more secure but less usable day to day since major apps don't work.
And everything is neatly locked into individual user profiles.
Remember, Verizon was a staunch proponent of CDMA adoption across the USA because it ensured vendor lock-in and non-portability across carriers. Yay capitalism!
EDIT: Also, Verizon has no plans whatsoever to shut down their LTE network.