I have to come on here and comment as I too was looking for a solution here. My friend has an iPhone 5 and disabled iMessage as a result of him not being able to message his wife anymore who had an iPhone but switched to Android. It made it so he could text anyone under the sun, but I could NOT send him texts despite him being on his iPhone with iMessage disabled.
And Apple should be smart enough to know that sometimes people's phones break, and then they could go a different route at that point, like my other buddy. His iPhone got water damage and he didn't have the money for a new iPhone so he went with a cheap Android. As a result I cannot text him without holding down the blue message bubble until the popup appears and selecting send as text message. I work in software development and can honestly say, it's not hard for Apple to implement a system that confirms that the device you're trying to send iMessage to is communicating via iMessage. Especially if a message is going to fail via iMessage. Why wouldn't the iPhone automatically send via text messge and see if that works instead of just giving you an error?
I love my iPhone, but come on this is such a simple design flaw that could be fixed in a small iOS update. iMessage-> send message via iMessage-> Check for iMessage compatability-> iMessage compatability not detected-> Revert to text message.
So don't blame a user for not knowing they need to turn off iMessage before changing phones.That's ridiculous. It's even more ridiculous a point because that does not even work! It does not alert the Apple server that you're no longer using iMessage on that number, and if it does, it's not doing it very well per my first example in this post.