My friends used to use a mix of iMessage and Facebook Messenger, while my extended family relied mostly on Facebook Messenger + SMS.
I didn't like this, I didn't have/or want an iPhone, meaning I was excluded from certain iMessage groups and I hated using Facebook so I was implicitly excluded from discussions via Facebook Messenger. Most of my communications were over SMS as a result, so not ideal for my social life.
Seeing this as a problem I researched alternative message apps that had feature parity with iMessage, I figured any attempt to get people to switch would fail if I couldn't get this much. I also decided to bank on the latent frustration people had with Facebook, the company, meaning I had to scratch WhatsApp off the list.
I ended up with Signal vs Telegram. Telegram had the sleekest interface and good feature parity with iMessage. While Signal fell short feature wise, it supported SMS (at the time) and some of my friends were interested in it from a privacy angle.
Ultimately I decided to be realistic, so I scratched off Signal and chose Telegram. The goal was to get everyone to switch not just a few who were "interested", so feature parity had to stay the priority, privacy be damned. My pick was very important because I figured the likelihood of a successful migration would decrease with each attempt I made.
Finally having made my choice, I consulted 1 on 1 with my individual friends and family members who usually organize events, and convinced them to install telegram + join my premade group chats. I then nagged them to notify everyone that all event planning would now be via Telegram and that everyone needed to install it now. I think the Telegram invite link really helped grease the wheels here.
Finally after setting the stage with that, I individually convinced each friend and family member 1 on 1, via a call or in-person, to install Telegram and join the new group chats. I made arguments such as: It would unify our communication under the same platform and make everyone's lives easier, we can use Telegram surveys to more easily schedule stuff, it has all the same features as iMessage + more, I already got X and Y to join so I really don't want you to be left out, I can help you install it, etc. I found it was important to take full initiative during all this.
Finally, in only a few days I got two distinct groups migrated onto Telegram. We have continued to use it for 2-3 years now so it's safe to say the migration stuck. The only one who I couldn't get to join was my cranky uncle who wanted signal instead (first I had heard of this from him), but his wife joined so it didn't really matter anyway, he is simply excluded from discussions now.
So ultimately, you can get people to switch if you put the work in :)