That's really not how it works. We’ve seen it before, when iPhones went from the “Dock connector” to Lightning. When Macs went to Firewire, and then to Thunderbolt. They will not have two ports.
And I never said USB-C will “get better.” My point is that it will be a better business decision later. When? Well, since the premise I offered is that people have Lightning already, and not USB-C and since it is well-understood that more people are getting more USB-C devices every year then clearly, over time, the cost for individuals to go to a USB-C phone will decrease over time, since they are more likely to have other USB-C devices over time.
It’s not really complicated.