Since they seem to have only delayed the deadline but not changed the terms, I assume all accounts from users who have not accepted the new terms will be deleted on that date.
That likely means not only no access to messages, but the deletion of them altogether - or at least I count on that but I don't have a way to verify it.
They still have the phone numbers of the people you're in touch with, they have the groups you're part of and all the phone numbers there. To get (most of) your contact list, they just need to cross that information with everybody else's contacts list.
Your not sharing is not worth much when everybody else is sharing.
On top of that, they also know times you're online, potentially location, who you're messaging with, who you're making audio/video calls with and for how long, etc.
Though importantly, if I didn't opt-out before, WhatsApp was already sharing this information with Facebook.
Right now I don't allow WhatsApp access to my contact list. It means all my messages are to phone numbers, which is a little inconvenient, but not overly so.
Come May 15th, will my app just refuse to open my messages list unless access to contacts is granted?