Whatsapp is used (at least here in the EU) for a lot more than staying in touch with your friends. E.g. having a young kid, it's used for: the class' group, arranging playdates, etc. Not using Whatsapp makes it much harder to arrange your social life and that of your kids.
Then there are also many organizations/companies that use Whatsapp to set appointments, for chat support, etc.
In many EU countries Whatsapp is pretty much replaced SMS. Only a small minority of folks have Signal or Telegram. iMessage is probably the only other thing that shows as a blip on the radar, but only a portion of the population has iDevices.
I agree that this is a bad situation, but WhatsApp became popular when it was still independent and their profit model was charging 1 Euro per year (which was much cheaper than SMS). Now abandoning Whatsapp is difficult due to network effects.