According to a post on WhatsApp’s FAQ page, it seems that by declining to accept the changes, your experience will be crippled rather substantially. WhatsApp claims that they extended the implementation of the privacy policy to give users more time to review the changes, but note that if users still do not accept these changes, it will affect the way they use the app.
The company says, “If you haven’t accepted by then, WhatsApp will not delete your account. However, you won’t have full functionality of WhatsApp until you accept. For a short time, you’ll be able to receive calls and notifications, but won’t be able to read or send messages from the app.”
It’s interesting that they say “for a short time” because it leads to questions such as what will happen after an extended period of time if users still do not accept? Will the company eventually stop users from using the app entirely, or delete their account? That remains to be seen, but in the meantime it seems like users will have to either accept it or find a different app to use.