I have the following products:
(and old iPads)
Kindle PW on order
Roku (not made by Amazon, but used for Amazon Video).
In terms of ecosystem constraint, I think you are really only meaningfully constrained by apps. Media constraints are less significant (although there is a convenience factor).
WRT media, the vast majority of music on my iDevices came from my CDs; I have a smattering of songs I bought from iTunes and a smaller smattering of songs I bought from Amazon. Neither of these companies use DRM, so the songs could be played on any future device.
The majority of my movie watching comes from Netflix. I've rented movies pretty much evenly from both, although I'm probably tending more towards Apple at the moment. Although if you have any time to plan, Redbox DVDs are by far the best deal.
I read exclusively in Amazon formats, although of course that could be changed if necessary. My long form reading is done on my Kindle, but for shorter works I use the Kindle app on my iPad or iPhone. (For whatever reason, I read more on the phone than on the iPad).
If I ended up getting an Android or Windows tablet and phone, most of this behavior wouldn't be affected; I would still be able to read my books on Amazon, play my music, or rent movies. And Redbox would probably still remain the best deal...
But for apps, like programs for traditional computers, you really are locked into an ecosystem. If I switched to a Mac, there are a lot of programs I would have to rebuy, and there are likely programs that wouldn't be available. The same is true of iPhone apps - if I switched to a windows or android phone, I'd have to rebuy the ones I wanted. Luckily, apps are so cheap (and the most useful ones are often free) that that's not really much of an economic loss.