I just say "can we use Signal? It's super secure." And people generally are fine to use Signal. They don't use SMS because it's expensive and many that own an iPhone in countries where Android dominate don't use iMessage because, well, few around them use iPhone and the interface (i.e., the phone number) doesn't really tell them that they won't get charged for a text message.
I've posted up in many countries, and it's rarely a show stopper.
Mine are annoying too, but I keep them because they also keep me in the loop.
If they are really your only option, you can use whatever insane platform they are on long enough to forge a couple friendships, explain your stance and then quit.
If they are your friends they will understand, if not it wasn't worth the effort to begin with.
My life is so annoying as a result of you, now I have to buy and maintain another phone, and then remember to check it, for your app.
Maybe, you could use something we agree on, like <insert plethora of choices>".
Most people will not, in fact, understand why you are so picky about the chat platforms you are willing to use, and they will think (correctly, I would say) that you care more about your social media stances than you care about their friendship. So you will think they were not worth the effort to begin with, and they will think the same in return, but ultimately they are not restricting their potential friend pool nearly as much as you are. If it works out well for you, great, otherwise you have to be a bit more pragmatic.
Poor reasoning. "We should go do hard drugs together, we're friends, right?", "You should just let me have sex with you, we're friends right?".
Using friendship as a cudgel is disgusting, and quite frankly, unfriendly. The above examples are extreme, but there is a cost to everything.
Pragmatic-ism cuts both ways.
It is perfectly fine to value certain things over friendship. You just have to be conscious of the friction that your convictions will cause and ask yourself whether it's worth it for you. Health is worth it. Chat apps? I don't know. If you ask me to use Signal, that seems reasonable enough and I'll do it, but if we're a group of 10 coordinating over Whatsapp and you want me to tell you whenever plans are being made, it's like, OK, but I'm doing the effort for our personal convos, can't you meet me halfway or something? That's the kind of friction you're going to deal with. Up to you.
If the content is compelling enough you will chat where you both agree to. If not it is like telling someone the only way you will talk is in the park by the chess tables.
Sure maybe you talk to them by chess tables sometimes, but if they won't meet you for coffee then they aren't your friends, just chess table acquaitances.
Real example: My friend group has a hiking group on WhatsApp and there is one friend who is not on it. One person will propose a hike, and will keep messaging the group with information about it. Someone then has to message that other friend and tell them about it, every time there is a slight adjustment to plans.
Let's say you are my friend, and Bob is arranging a hike for 10am on Saturday. You usually come on the hike so I let you know. Even though all the details are on the group, you ask me all the questions and I have to fill you in. Then on the morning of the hike Darcy's car breaks down and the time gets rearranged from 10am to 11am, so then I have to tell you that as well because you aren't on the group chat, and suddenly I feel like I am becoming your PA (and if I forget to tell you the time has changed then suddenly it's my fault - even though i'm not arranging the damn hike and you could have been on the group in the first place you just refuse).
Ugh, I might but it's annoying for me (your friend) to be your personal messenger whenever there is something you might be interested in on the group chat.
Why is it on me to keep you in the loop? It's why we created the group chat, Dave.
Particularly that time when you got all annoyed at me for not being invited camping - I didn't even organise the trip in the first place, it's not my job to tell you about every event on the group chat just because you won't download WhatsApp.
Absolutely increases it's stickiness.
It's not perfect: when you connect with people they can easily associate your WhatsApp account with your identity (add contact) and also share info/pics/etc about you that you wouldn't want shared. They can do this even if you're not on WhatsApp, and Meta can use face recognition and other techniques to create a profile for you, then associate that profile with the other days and profile they have for you. It's trivial to narrow the down to a few million users then match on photo etc. Unclear if they bother, given that 99% of users just give away their privacy.
Also, Whatsapp verification uses SMS. If they'd used a verification URL, they could've (bounced through) Facebook.com, Instagram.com, etc and associated the account via (first party) cookies. I'm guessing they don't because 99% of users happily give the app all the permissions it needs and SMS is more universal.
In countries where WhatsApp is ubiquitous, it is used for more than just messaging friends.
For example, many childcare places will send updates through WhatsApp groups. So you have to convince the business and all their clients to switch to signal or do without notifications for your child's childcare.