Not sure why you have chosen to post under switched on families?
VM are a commercial company and set pricing for new and existing customer deals is based on what their data indicates is likely to maximise profits. Companies only reward loyalty if it increases profit. In the same way it is up to every customer to decide whether to stay or leave when they reach the end of their contact.
Bye the way, did you use the thinking of leaving us option when trying to negotiate a better deal?