Nonetheless many of us don't want to be tied to Amazon and don't want to be sideloading. The library lending is really useful.
I still have the Sony PRS-350 I bought years and it is still going strong. At the time it was much better than the Kindles with the touch screen and smaller size, obviously now other devices have moved on.
You however are missing my point. Whatever route you go you either have to use the default store (if the device has one) to purchase/get your books directly to the device OR
you are have to sideload it from your computer so you are in the exact same boat.
And the problem with most of the other ebook stores is how reliable are they in the long term. Look at the issue people had with Digital Music where you had all the competing devices and stores tied to these devices that. I bought music from a few different sources back in the day but the only ones I can still download are the iTunes ones (And even though I have all the source files still for the others they won't play as the licencing servers no longer exist)
So its fine if you make sure you backup all your books and ensure there are no DRM restrictions on them so if say the Kobo store shuts down you can still access your purchases (Kobo store is perhaps one of the best placed ones outside Amazon as its fairly well established but not sure its yet at a point where it makes enough ebook sales or its hardware profits are high enough for it to be totally secure. The high street retailers I wouldn't trust based on their history with digital music)
The likelihood of Amazon shutting down is alot less (Although I still take a backup of all my purchased books and make sure they have their DRM removed as I got burnt with Digital Music)