All I have to ask is why though? They already have access to skinned aosp and from there can(and do)quite a bit of tweaking on their own. Fireos has been a worse version of android for some time now and Im unsure what the benefit of making their own in house OS would be.

If it's a true GNU/Linux OS with compatibility with linux programs, then that would be kind of neat, and if it's open enough to let advanced users install flatpaks(I suspect it's going to be immutable so at least flatpaks would be nice) then that could be neat. Currently it's very easy to sideload on fireos devices and even install the play store in full so it's possible the end product could be more like the steamdeckOS which is very much a user friendly store front end with a power user true linux experience underneath.

That said, for some reason I suspect that they will be locking things down even more and its going to be one of those many user facing linux devices that's technically linux but very limited. Like a smart fridge interface or something. If this is the case then dropping android support would be a bad move. You lose easy/lazy portability to your store from developers who already have a product to sell and you lose many apps that already exist, and for power users you lose access to the many apps that can easily be side loaded like tachiyomi(though I imagine amazon would rather you buy from them than buy their subsidized $80 tablets to read pirated manga/comics and library books on libby)

But who knows if they actually do an OK job this could lead to a new wave of GNU compatible touch forward apps for the rest of us. Linux has gotten a lot better at touch forward design over the last 4 or 5 years on its own, but its still fairly rough.