Sometimes I really don't get your paper doubt resistance
USB-C would make plugging in easier and you said it yourself, the Kindle comes with a cable anyway (no problem if the side of the cable that plugs into chargers is still USB A) and sooner or later, the USB C port will replace micro USB and others.
What difference would it make then if the Kindle came with one now? You will have to give up your micro USB collections at some point...
Also, the fact that USB 3.1 is not supported by modern ereader SoCs is no problem either. The connector can easily be used to support USB 2 or 3 speeds.