At what point do phone numbers become obsolete? Considering that I basically never call anyone, or receive calls (I do use voice communication but it's all voip), and that nobody where I live now uses SMS, what's even the point of that number?

Don't think it will happen soon. Phone numbers have the advantage that they are harder to misuse than email addresses while being partly anonymous and company independent. Furthermore, you can provide some identification without using another password. Nearly everyone has one and you have a 1-1 relationship to (most) phones. I actually think that phone numbers will become more important with more apps using it as an account (e.g. Google Duo).

I hope that never.

Getting a prepaid SIM card with a number I have power over to disclose to only whomever I want is still great in my book.

Except that there's a huge security whole (SS7) in the phone networks and it's been there for years. https://www.youtube.com/watch?v=dDCYlGX22Q0

Maybe they could use WhatsApp to reach out to us -- ie. what used to be an ad on facebook will be a private message via WhatsApp telling you about the latest and greatest from your favourite fast food restaurant.

On a different note, they keep saying that nobody except the parties directly involved are able to read the e2e encrypted messages. They could be lying, couldn't they? Who knows that they (or somebody else) doesn't have a master key?