iMessage's killer feature is that you can send a group message to more than a total of ten people. SMS does not officially support this.
I do not recall this being a problem in the pre-iMessage days, even with flip phones. I was on prepaid T-Mobile and had to pay for every message I received, which is the one place where I thought the US billing system had gone nuts - you could always decline to receive a phone call, knowing the number, but you can't refuse a text. I was surprised that nobody ever abused that, but they never did so on a large scale. At ten cents per text, sent or received, you communicated when you needed to, or when you had something to contribute, but one minute of voice was the same cost. One sent and one received text cost as much as two minutes of talk, which can convey a lot more information.
I can't remember the last time I sent an SMS. Comparing iMessage to SMS is not useful. Compare it to one of the (several) cross platform messaging services.
In Europe, Android is on equal par with Apple. WhatsApp is the default messaging service - it is universally installed among my friends, relatives, colleagues, contractors etc.. No-one cares what model of phone the other person has. Everyone communicates.
I do not recall this being a problem in the pre-iMessage days, even with flip phones. I was on prepaid T-Mobile and had to pay for every message I received, which is the one place where I thought the US billing system had gone nuts - you could always decline to receive a phone call, knowing the number, but you can't refuse a text. I was surprised that nobody ever abused that, but they never did so on a large scale. At ten cents per text, sent or received, you communicated when you needed to, or when you had something to contribute, but one minute of voice was the same cost. One sent and one received text cost as much as two minutes of talk, which can convey a lot more information.