You don't persuade them. They've already made their decision. Now you have to make yours. Their reasons for not wanting to switch are just as valid as yours for wanting to.

So, you either switch and accept that some of the people in your life don't actually care enough to come with you, or you're the one that has to adapt to multiple apps to communicate with others. That's really what it boils down to. Most people don't care about the matter, and there's a segment of people in most of our lives that don't care about us if there's any inconvenience involved.

Some of them made alternate suggestions, which means they're willing to go through some inconvenience for you, just not the specific inconvenience of having an app that only you and they will be using.

Despite now having storage space for multiple messaging apps, people resist the idea of having more than whatever arbitrary number they've decided doesn't work. In some cases, that number may be one. And the truth is that remembering who is connected via what app/service is a pain in the ass if there's enough people in your life. Some people can't handle that memory issue and are just going to refuse outright out of necessity.

So, stop trying to change their minds and seek compromise. If they're willing to switch to telegram, you can at least have some degree of encryption, so go with that for anyone that's expressed willingness. Let that core group become the reason for anyone else to join in.

Unless you just want to play hardball and refuse to communicate with anyone on anything but your choice. There will be some that cave and join in. But you'd be amazed how many people and which people don't really want to talk to you enough to do so. But you'll have a small group of people that are now using it with you. You'll have to help them get set up, and be prepared for the inevitable tech support you're volunteering to provide, as well as the need to guide them through the learning curve of it.