In the long term, I think that the success of iMessage apps will be limited. Not only is the experience of using iMessage apps fraught with confusion, I also think that in today’s App Store its difficult to make money on single purpose utilities, which is essentially the nature of most iMessage apps. iMessage Apps may make great complements to traditional apps, especially if the experience of accessing them is improved, but I think that they are difficult sell on their own.
However, I have a more positive outlook on the long-term sustainability of sticker packs. I think that its possible for indies who have a great breadth and depth of sticker packs to make money from them, but that long-term sustanability is contigent on iAPs being available for sticker packs.
If Apple makes iAPs for sticker packs, and sticker packs release more stickers in the form of iAPs. Even if the initial conversion rate for new iAPs is small, I think as those percentage of converters use the iAP, the fear of missing out will perpetuate, resulting in more conversions over time.