My primary concern was that your IM-to-text plan wouldn't work where coverage is lacking. If I'm somewhere that has no data network but does have signal then your setup is at a considerable disadvantage. I don't know about you, but it seems pretty often that I find that I have limited or no data coverage but reasonable signal strength, especially in this backwater village you call a town.

Crazy apps that run down battery life- Grindr (OH GOD WHAT HAVE I BECOME). If it is any indication, battery may be a concern.

Also, incredible profit margins for texting doesn't make it less justifyable; if anything, it makes it more justifiable, because in a capitalist system making more money is better. I'll grant you the no-access from any other platform argument, but you can say the same about phones, which is sort of the point. If people wanted a way to access their texts from other sources, they wouldn't text, they would email.