Almost everybody I know has at least 2 messaging apps installed on their phone. What's to stop people start migrating to Signal/Telegram/Viber/Keybase/etc.. once WhatsApp starts displaying ads?
The same thing that is currently forcing them to use more than one chat app, no matter which one they prefer: Network effect. If you can't convince all your friends to switch at once to one system, you'll be stuck between a rock (e.g. ad-infested/data-leaking WhatsApp) and a hard place (friends reaching out to you much less frequently because it's a PITA to reach you).