Apple’s m.o. early on was a take it or leave approach with the carriers. Usually the largest carrier in a market would refuse their demands but a smaller carrier would acquiesce. Then the larger carrier would start losing customers since they didn’t have the iPhone and they would come to the table.

What do you mean by incentivise? Either manufacturers delivered custom firmware or they carriers didn’t sell their devices, period. Apple changed all that.

I see. That makes sense if you upgrade/buy through your carrier. That isn't something I've ever done so my carrier has never been a limitation on the phones I can get. I guess Apple made the carriers realise that everyone will get the phone they want with or without them in the purchasing cycle.

Most people get their phones from carriers.

Why would you want to buy a device from a carrier ? Just buy a phone from wherever and pop in a SIM.