There is not iPhone that is Lightning to USB-C only. You can, in fact, use a USB-A cable.
And yes, common sense (something that is often lacking in today's world) tells you that you should buy a USB-A cable if you lack a USB-C charger (instead of buying a new charger).
The reason Apple went with USB-C is that it is evolving as the new standard, and is what is typically included with most new devices. If Apple had included a USB-A cable people would be moaning that Apple is building in planned obsolescense.