Almost no one is using USB-C currently. It's hardly a "standard". And USB-C audio is a long way from becoming mainstream. The reality is that whether it's USB-C or Lightning, adapters will be a necessity for the next 10 years to ensure compatibility with all current audio products, equipment and devices, so USB-C or Lightning is likely moot. Apple users on the other hand will likely enjoy complete compatibility across all new Apple products going forward, as Lightning will be added to Macs.
And by the time that USB-C is widespread enough to actually qualify as a "standard", Apple will most likely be dropping ports entirely from their mobile products going with perfected wireless charging, wireless audio, and wireless data transfer.
I wouldn't be at all surprised if USB-C never becomes a standard, because it is replaced by something newer, smaller, more durable, more efficient, and safer. Meanwhile, Apple customers who have amassed a whole new collection of Ligthing cables and accessories in the last 4 years, since they had to replace their 10 years worth of 30-pin dock cables and accessories, will not have to shell out even more to update everything all over again to USB-C.
There's no question in my mind that Apple won't be replacing Lightning any time soon, especially after doubling down on the Pencil, TV remote, and Mac desktop peripherals.