It would make it a hurdle for the young userbase. When I started using Whatsapp I was ~16, and didn't have access to a credit card or any other way to pay for a yearly subscription (and I'm not sure if my parents would have okayed it).
WhatsApp made every year free for 90%+ of Android users. Maybe 95%+. It is hard to find people who paid for Android Whatsapp. Even more so paying more than once.
