Most people that own iPhones and other iOS devices have tons of Lightning cables already. Moving to USB-C just to consolidate cables would be a waste for most. I can see the benefit of being able to access USB-C thumbdrives and HDDs, but do you think Apple would really allow that on an iPhone? The iPad is a different story as it's being billed as a PC/Mac replacement.Benefits of replacing lightning with USB-C, hmm... The ability to plug in a USB flash drive and transfer all of my files easily to a computer. I only need to carry ONE charger that can charge an iPad, MacBook and an iPhone, while still being compatible with my friends Android devices of one of them forgets a charger. Makes it much easier to interface with a MacBook if I only need one USB-C cable for both an iPad and an iPhone.
There's really no benefit for moving to USB-C at this point, at least not yet. You won't get better audio or video outputs. At least when they transititioned from 30-pin to Lightning, there were tangible benefits. Connector durability/reliability, higher throughput speeds for syncing, smaller connector, etc. I don't really see much of a benefit for switching the port.
Arguments for switching to USB-C
1. Compliance with EU desire for standard charging ports across all mobile devices
2. Ability to connect to mass storage devices
3. Convenience for users that have already transitioned their devices (Mac/PC, new iPads, Android) to USB-C
4. Compatibility with new accessories designed with USB-C
5. No Apple royalties, therefore accessories would be less expensive, in theory.
Arguments for staying with Lightning
1. Lightning is a smaller port
2. Lightning is more durable (at least to the naked eye)
3. Huge installed base of Lightning users
4. Huge installed base of Lightning accessories
5. Apple receives royalties for Lightning (I suspect that many don't care about this)
Personally, I don't really see Apple switching to USB-C for the 2019 iPhones. Perhaps they'll wait for the installed base of USB-C Macs to grow (with the MBA now sporting USB-C) a little larger. 2020 iPhones, I'd be more willing to guess would be the year.