WhatsApp doesn't use libsignal, and Android is already pretty Rusty and deployed more than WhatsApp around the world (not just smartphone. Tons of "embedded" use cases also run on custom Android)
>deployed more than WhatsApp
If you count old Android versions before Rust was added.
Like our gym devices that have a full tablet to run a basic application to control weights, talk about wasting money.
WhatsApp was using libsignal (the C version) when I worked on the KaiOS integration in 2017/2018.