The best way to get your friends to switch messengers is to not be reachable on the bad ones: ie delete your accounts and uninstall the apps.
Retaining your account on bad platforms makes those platforms more attractive to other users and makes the world worse.
Even niftier: In some countries Facebook created this "FreeBasics/internet.org" scheme where access to Facebook services is free. When travelling Indonesia WhatsApp was the only way to reach the landlords of guest apartments, taxi drivers, ... not even regular SMS/text.
The strength of the network effect can be hard to break.
When you cave and install WA, you make it harder for the next parent, just as all the ones before you made it harder for you.
If you want to be rid of these things, you can't get there by using them because you think you have to.
The vast majority of people don't think Facebook is the abhorrent service it is, and an even larger majority don't realize WA is owned by Facebook.
That's not true. A lot of people are a lot more concerned about their privacy these days.
You can obviously argue that they can't siphon away information at the server level; but if you don't trust whatsapp/signal/telegram at that level then you're boned anyway because they could quite easily activate the 'whatsapp web' function and exfiltrate information via whatsapp servers.
Ideally we'd have clients and servers and server operators as distinct and uncoordinated individuals, which would have to work in conjunction with each other to unravel your messages: but there's no incentive for them in place to do that.
I, a server operator, can pass encrypted traffic, that data is encrypted with clients I don't/can't control.
I, a client software developer can make encrypted traffic, but messages can only be passed on networks I can't control. (and if it didn't, this would be easy to spot).
I, a server software developer can write software which allows traffic to be passed but does not promote an operator to exfil even encrypted data.
We have this with Matrix, IRC, XMPP etc;
IMO if you're going to use Whatsapp; it's the same as if you use telegram or signal; I understand that this opinion is controversial but you're choosing convinience over security and E2E doesn't mean anything if you have one entity controlling the ends.
E2EE is not the main concern any more for most though. Most messengers offer some form of it, and you're correct in saying it probably won't prevent the company from accessing your data. However, I think the point is about which company to trust with this sort of power. Actually, it's more like "which company is least likely to do [bad stuff] with my data?"
Currently, facebook is much higher on that list than the signal foundation -- at least in my book.
But it has a native bot API and can handle multiple clients on multiple different devices without any issue.