Whoa!
Interesting, and makes engineering sense, but still big news. 
That would kill the platform for me…
And most other people.
A good way for Amazon to shoot itself in the foot.
I doubt if most of the millions of fire TV device purchasers have any idea whether it’s android or something else. Samsung has done OK with their Tizen operating system. (not saying it’s great, just that they are one of the top TV sellers in the world. It hasn’t hurt their market share.)
Tablet sales might be a different story…
IMHO, Tizen is actually better than what they had before so for TVs I considered it a plus. Way better than that dogs breakfast of a mess LGs WebOS is.
However, Amazon tablets are bad enough now with the bastardised old version of Android they run let alone some bodgy crippled Fire OS…