Nobody buys it.
Do Linux phones have access to WhatsApp? If not, there is no point. It will be a perfectly good mobile computer but it will never actually replace the one in my pocket. And that's coming from a programmer who loves Linux. Imagine the utter disdain normal people would have for a phone that doesn't even run WhatsApp.
(Sailfish OS is a Linux based mobile distro.)
In any case totally evil behavior from their side. Its already bad enough they run totally proprietary centralized service with no public API & ignore any non-mainstream mobile platforms. But when they start banning users of third party clients on those platforms and go after third party client developers with presumably legal treats - that's pure evil, no way around it.
How commercially successful it'd be is still a different story, but it could run WhatsApp.
For another, SafetyNet is used primarily for DRM on streaming services and for banking or other financial apps. Absolutely no messenger uses it that I am aware of (unless you count Snapchat as a "messenger" but that's literally it).
Every FB owned app runs fine on Android devices that fail SafetyNet. No reason to believe that'll change because where's the benefit for FB?
I do, and it's pretty popular, so I don't think you should discount it so quickly.