Epub is not KOBO format, it is a format by Adobe and is pretty well accepted, so DRM is not going tobe a issue even if Kobo did fail.
Amazon uses their own system and as poplar as Kindle is, a lot of other stores just don't offer ebooks in the kindle format
Epub is not an Adobe format at all. It's an extension of the open ebook format. Amazon purchased the Mobi format a while ago and used that as the basis for the Kindle format. Whether Amazon charge others to use their format or not, I dunno.
As someone who has created and edited ebooks in both formats, epub is the easier to work with.