That's a respectable amount of money! I assume that's annually. I really like plans where funds are valid for a long period of time (ex. 1 year). Although many people will use it much faster, there's a certain peace of mind that goes along with knowing that funds won't disappear before you get a chance to use them.
Although I understand that there's a certain network carrying cost even when customers aren't using the phone at all, and that they can't lose money, I think most users will end up staying with them longer and being profitable overall.
Particularly with the available unlimited plans, I expect people can save even more by going prepaid. Especially small business owners who are paying for their employee's phones and don't want to worry about overages. That T-Mobile plan really sounds like a great offer as well!