I hope they stay with lightning cables because there literally everywhere. I go to work and guarantee someone will have a cable I can borrow. If I visit friends or relatives there will be a lightning cable I can use. I had a note 9 and trying to find a usb-c cable to use was a no go no one had one.
It really depends when you plan on upgrading your phone if Apple decides to transition to USB-C. If you plan on retaining your phone that is obviously lighting enables, then you really have nothing to worry about at this point. Over the course of time, The transition is coming, however; The real question is, does Apple make the full transition to USB-C for the 2019 iPhones, or do they provide adapter in the box.