Apple's iMessage messaging service will never be available for third-party platforms. However, at one point, Apple planned to do so.
According to Scott Forstall, former head of iOS, Apple tried to convince carriers to include the best features of iMessage to create a standardized messaging service that would be available to everyone. However, difficulties were not overcome and the plan remained unfinished.
iMessage brings a lot of features to iPhone users that aren't available in the text messaging service available on other platforms, like Android. These features also help Apple retain its users, as it seems that once someone uses iMessage, it's hard to switch to any other messaging service.
An article published by The Wall Street Journal, by journalist Joanna Stern, described the role of iMessage in keeping users in the Apple ecosystem. The article highlights the problems users face when trying to switch to Android.
According to Scott Forstall, Apple “reached out to carriers to look at adding features to existing text messaging systems and eliminating additional costs for the customer.” They wanted to bring the best of iMessage to every platform, but carriers refused to cooperate.
“For a variety of reasons, from the difficulty of extending existing standards, to challenges with interoperability between text messaging systems and the desire of providers to protect a significant source of revenue, these plans did not materialize.”
Apple could certainly develop an iMessage Android app, but that would open the door for iPhone users to switch to Android phones, which Apple doesn't want. Therefore, it's safe to assume that the service will remain exclusively on the iOS platform and never be available for other platforms.
