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Apple is big enough that they could make the carriers keep SMS. If a carrier tried to remove it and break "compatibility" with the iPhone, Apple would block that carrier from having iPhones altogether, which would decimate that carriers business. So through a causative chain, Apple could force carriers to keep SMS.
a) Apple doesn't decide whether carriers keep SMS. Governments do.
b) Apple won't stop users from using a particular carrier because of commercial considerations. It would be a clear breach of anti-competition laws.
c) Many carriers don't have relationships with Apple to sell the iPhone. And in many places internationally the iPhone is not even that popular. What matters is whether you can buy a SIM and install it into your phone.