1) OnStar is a hardened part of the vehicle, built specifically to endure a high speed collision. Someone's smartphone in the center console would likely be ejected or demolished by the same.
2) The two way voice phase before initiating emergency services gives comfort that there aren't false emergency calls that can get you in legal or financial troubles.
3) Via the antenna on the car (raised, on the outside of the vehicle, large), I can confidently say that OnStar has given me connectivity in many cases where my smartphone has none.
I'm not trying to be an ad for OnStar, but based upon those facts I can't make the economic decision that this is a replacement for that. For someone who doesn't have OnStar as an option this would likely be fine, but just thought it was worth mentioning that.