It turns out the budget shows the phone number registration problem: the costs to deal with phone number verification seem to be $6MM, which seems to be 10% of the entire budget.
If Signal staff are reading this, I'd gladly pay $100/year for a phone-free solution for all users.
You might understand that it's a bad idea, but that makes you an outlier.
This is why it still has a stronghold as well…
Given the choice between SMS and a service that provides the same functionality is free, superior in most ways, borderless, etc. the choice to use whatsapp is obvious.