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.

No Kindles actually support reading ePub.

All that's happening is Amazon is retiring MOBI support. Since Send to Kindle only supported MOBI, they are transitioning Send to Kindle to support receiving ePub which then gets converted to AZW3.

So the only Kindles losing support for the Send to Kindle stuff is the Kindle 1, Kindle 2, and Kindle DX. Since those ones never got AZW3 support.

Anecdotally, I have sideloaded via USB EPUBs and it works on two of my three kindles. Again, I don't know where the OS line in the sand is for EPUB support, but kindles seem to have quietly updated to directly support EPUBs. Amazon could do a much better job of communication here. I assume they aren't communicating this well because they don't want to encourage people to use non-Amazon bookstores.

If this is true you should report it to Mobileread, because this would be a giant story. I have never heard of that being possible, nor has anyone else there. Are you completely sure that you actually sideloaded the ePubs and not AZW3 or KFX files?

I feel somewhat certain I spotted people already doing it on a forum like Mobileread (or maybe it was Mobileread) and that is what encouraged me to try it as I was fiddling with something in Calibre conversion that wasn't working right and searched for "obvious solutions" one of which was a nonchalant "just try to copy the EPUB file over USB". I don't know what to tell you about why it isn't a bigger "story" other than "it seems to be intentionally quiet on Amazon's part" and I guess it is the sort of thing people just kind of do and don't think to talk about?

I personally use `ebook-convert` (comes with Calibre) with a simple script that'll capture anything in a drop dir, convert it, and email it off.

You can always convert EPUB to MOBI yourself using Calibre. I often have to convert the other way (EPUB to AZW3) with a newer kindle.