My understanding is that Amazon already crossed into that "mixed" state you are worried about: after a certain OS version Kindles natively understand EPUB (and can be sideloaded via USB, not just emailed). This was rolled out relatively quietly and the Send to Kindle tools have been taking a lot longer to update to support the .epub file extension even when the devices support it. We have something of an idea of when it rolled out in that it is noticeable the current "modern" KFX (?) format is a light tweak of EPUB.
But there are indications that there are probably still active kindle devices which Amazon is converting AZW3, KFX, and EPUB behind the scenes still. I still have a 3rd gen kindle keyboard myself that I use rarely and the outward differences between its OS and the OS on my other two more recent kindles are growing increasingly stark (and not entirely for the better, Amazon's recent cover focus even in list views have made the device's library a much bigger chore to navigate with no more than 6 books displayed at a time in either grid or list views). Easy to imagine nearly as many inward differences, including format support, between the OS versions.
So far, Amazon seems to want to support any active kindles for the long haul and I would be surprised if any behind the scenes converters get "turned off" without notice, but because they don't tell us where the exact lines are in which devices support which OS versions and which OS versions introduce which feature support, it's a strange gamble with not enough information.