In a nutshell, WhatsApp will take your toys away unless you play nicely. You’ll lose full-functionality and be restricted to receiving calls and notifications and won’t be able to read or send messages. You can still accept the new policy after the May 15th deadline, and WhatsApp says that it won’t delete accounts if you don’t do so, but that its inactive user policy will apply.
You can download your chat history on Android or iPhone and a report of your account before May 15th, and move to a competing app. If you choose to delete your account then all your message history will be erased, backups will be deleted, and you’ll be removed from your WhatsApp groups. WhatsApp says that account deletion can’t be reversed.
Personally, I’ll probably stick with WhatsApp as my Dad is comfortable with it and most of his cronies are on it, but I plan to dip my toe into the Telegram/Signal pond over the next couple of weeks with other contacts to see how they compare.