For example, in Spain, people will not communicate in written form with you outside Whatsapp. You can try sending an SMS, they won't answer to not risk being charged.

Your friends will communicate and organize events only on Whatsapp. Your colleagues will have social interactions on Whatsapp, even send you work messages when on the go. Your landlord will communicate via Whatsapp, including sending you utility bills to pay. The person you want to buy second-hand stuff from communicates via Whatsapp. The real estate agent trying to find you a place to live will send proposals by Whatsapp. Many businesses advertise customer support on Whatsapp also, although in that case it's never the only option.

It's not literally impossible to live without it. I know some grandparents without smartphones. But it does make your life a lot more difficult.