£.99 isnt a lot to pay yearly for whatsapp but its more the principle behind it. I initially bought the app as it offered me a lot of amazing features for a one off fee. Having to then be expected to pay a yearly subscription is somehow a breach of the original contract.
Anyhow having read further, it seems the subscription model only appllies to new users
http://www.theverge.com/2013/3/18/4121940/whatsapp-for-ios-moving-to-subscription-model-this-year
Regarding those that say its only a pound/dollar, think about it, what if all apps then decide at some point to go the subscription route, will you still think its only .99p when been charged yearly for 99+ apps?