WhatsApp is based on XMPP, hence why it works so well, but with the difference that nobody "owns" XMPP (it's a network of account providers, just like email where you can be a @outlook.com and exchange messages with @yahoo.com).

You can start there and see how well it works for you and whether you want to embark your contacts with you. For me the main motivator is that XMPP, as a decentralized and extensible protocol, will still be there long after I'm dead. I'm done telling people around me to install this or that App, all I want is the confidence that whatever I'm using is the last messaging system I will ever need (won't go bankrupt, won't turn against me, will remain secure, will have all the desired features ...), and that's the only one that qualifies.