How will they open up iMessage? It uses a bunch of iOS specific features. If they ever did, they would have to strip it of a few of the features that only work on Apple products and it would be a separate app that you would probably need an Apple ID to use. Like how you need a Facebook account to use Facebook Messenger. At that point, it would just be like a worse version of WhatsApp. It would be pretty pointless, unless you really want to have a blue bubble with iPhone friends.