A Linux phone could run WhatsApp if the Linux distro was implemented in such a way that it could run Android apps (remember Android is basically just a runtime on top of the Linux kernel) and since AOSP is FOSS this is very doable and legal.
How commercially successful it'd be is still a different story, but it could run WhatsApp.
How commercially successful it'd be is still a different story, but it could run WhatsApp.