[ApK]

Do keep in mind that "but I'm still reluctant to limit my purchases to Amazon" is not really the issue. There are other sources for books for Kindle besides Amazon (Smashwords, O'Reilly, Baen, direct sales sites, libraries, PD sites, etc).

But presuming we're talking only about "large mass-market DRM ebook sellers," on a practical level, that means "B&N, Kobo, and Google."

Apple is a different system, Sony is out, and while I may be forgetting one or two ADE-based contenders at most, it's not like you are ensuring a huge world of competitive options.
The pool is actually pretty shallow, and while there are deals to had by shopping around at all of them (thank you Alf and ilk) none of them can boast generally better selection or price than Amazon.

Might Alf's mojo stop working in the future? Sure. Can you guarantee that Adobe or one of the other device makers or book sellers won't screw you over similarly in the future?

Unless you just have a principle objection to patronizing one vendor over another, get the device you like best.

ApK