That's the problem with many proprietary consumer devices (take the iPad into this category as well).
1. Vendor lock: You can't install or change most of the things including OS and Apps.
2. Locked down OS: You can't do anything with your device. If you try to change the OS you'll maybe brick it.
3. Planned obsolescence built in: If the vendor decides it's "time to say goodbye" you won't get updates anymore and probably feature by feature your device won't work any longer. After some time it's so old, insecure etc. you don't even want to use it to do even the simple tasks it was designed for.
The kindle isn't a reference for me in this sector at all. If you want to start the discussion about nice e-Readers you should consider comparing the Sony DP, Onyx Boox Note and the Remarkable. Those are the leading innovators and so far I'm afraid I haven't seen any device that can compete.
Personally I'm waiting for an eInk device with color (at least some colors), wacom pen that works without battery (like in Onyx & Remarkable) but an open, real linux on it so you can do anything with it.
The Onyx comes close to this as it uses android but as we all know it's not really the linux you want when you wish for a "real linux" ;-)
I'm a bit sad that innovation in this sector stopped and I'd be willing to pay up to 1k $ for the described features.