Amazon made epub the default delivery format for publishers more than a year ago, so there was not much technical reason they couldn't do this.
For books you load into the Kindle Device.
Now they just need to actually release a refreshed version of the Oasis. It's been 3 years and it's really showing its age in features, connection and battery life.
My 7 years old Paperwhite is still going strong :)
Yeah, mostly I want a new one for myself. My old Kobo died and my partner's Oasis is nice, but I'm not going to buy an ereader with micro-b charging like a chump.