It's not that easy. Installing another messenger is easy, but you need to migrate your contacts to it as well. If you've been communicating through Facebook Messenger and don't have their phone number, you're shit out of luck. With WhatsApp you're somewhat better off since it uses phone numbers as user identifiers, so you at least have everyone's number, but international texting is incredibly unreliable and international calling is prohibitively expensive for lots of people.
