what would it realistically take to move an entire country off of whatsapp?

Wisdom.

Same as what it took to have it take the whole market: a better alternative and enough advertisement for a critical mass of people to migrate at once, so that they stay active there.

Then the rest will flock more or less progressively.

The first people have to abandon the idea that they’re only allowed to use one product of a category at a time. It will not kill you to use to chat apps, it will not kill you to use two microblogging services. The second is that you need to prioritize the better one when you can. This eliminates lock-in effects which are a major moat