So the hardware and the O/S is a little limiting? Fine. It isn't the type of device you need.
On the other hand, all of the ridiculous levels of settings and personalizations and all that on a 5.1.1 or 6.0 Android box is too much for my parents to want to argue with.
There's something to be said for "works well enough". As developers we should not forget that, lest a simpler product line come along and put your complex fully-featured super-product out to dry, no matter what features the new upstart is missing.
I'm not saying Amazon's Fire will do that to Android, as the tablet market right now is still pretty large and has room for all (and of course, Fire could easily re-enable Android features they've suppressed at a moment's notice should the demand truly be there). But it is something to consider that not all products are the right fit for all audiences.
I have a Fire that I use for reading at night (Feedly, Pocket, and Facebook - all the links i've saved throughout the day - and kindle books if i'm still awake after all that). It works perfect for that purpose (I also use it as test platform for my apps since i'm targeting that easier-to-use market).
But I take a Samsung Tab 4 with me during the day, because that's the better one for when I want interactive stuff or games or things that require Google Play Services and all that.
Right tool for the right job and the right audience.
(That said, FireOS 5 did have a few really annoying bugs I've had to work around, but nevermind... :) )