I will never convince my friends to use an internet-based messenger - I might as well try and get them to use Pidgin+OTR if that's the ball game. I have, however, switched a few of them over to SMSSecure, the SMS-only TextSecure fork, which is a perfectly serviceable SMS client.
I too think SMS transport should have been maintained, but I believe it only really matters when you're stuck without mobile data access (for example, when I was on a cruise ship earlier this year I had roaming SMS/voice but not data).
As long as you _do_ have mobile data access at both ends, then a message to another TextSecure user will be sent encrypted over the data connection; and a message to a non-TextSecure user wouldn't have been encrypted anyhow.
I currently pay €2 per month for my mobile bill. That is very little money. Notably, that includes no internet/data, but it does include unlimited text messages. Can you see why SMSSecure is something i would want?
By the way - the biggest problem with textsecure^Wsignal right now is the lack of encrypted export and import of the private key. Forget the issue about message history, the freaking private key is the most important thing. All the dumb users you may manage to reach with your current integrations and conveniences are just accepting a changed key for their contacts at any time, and now also smart and diligent people are doing the same thing because you can't keep private key continuity unless you root your android phone and use a root backup app. It's a basic core failing.