what's great and unique about SMS is you can send a SMS message to any cell phone and it will chime and the user will get a notice. maybe if you know that a person has snapchat you can snapchat your butt or whatever. but snapchat will die, so will everything else. sms as a technology isn't going anywhere.
True, the universality is a benefit, and that's why SMS will probably remain as a "baseline" service for quite a long time. OTOH, it *is* very limited, even by the standards of the late-90s when it first became *really* popular. (The 140 character limit is more reminiscent of limits imposed by the tiny RAMs of late-70s computers!).
Also, rather obviously, you can't SMS text a photo of your butt(!), and most of the end-users of other services are probably only doing so for ephemeral use- let's face it, that applies to text messages as well! In Snapchat's case, the whole *point* is that it's (supposedly) ephemeral. (Of course, I never trusted that as far as I could throw it, and apparently Snapchat were in trouble for retaining images themselves, which makes it even worse, but that's beside the point here).