The C1 modem is also not a feature that users care about, other than the increased battery life (that Apple does advertise). In fact, there will be concerns that the modem isn't as good as the existing ones in some edge case.
Putting the new modem into the new "budget" phone reduces expectations so that the impact of any issues will be blunted. Only when all the problems have been ironed out, that will be when the mainstream iPhones will get the new modem.
Putting the new modem into the new "budget" phone reduces expectations so that the impact of any issues will be blunted. Only when all the problems have been ironed out, that will be when the mainstream iPhones will get the new modem.