With the proliferation of Nexus tablets (or Android tablets in general) and that Play Books is stock in the OS, I feel pretty confident in it sticking around.

The point is, and I should have made it clear in my original comment, I would usually prefer one company that just does something rather than a corporation that tries to do everything. Their priorities change quickly; mainly because they have a lot of products and even if a company is useful to them and their users they might shut it down because it might be "relatively" less important to them. Whereas a company which just does that might hang on to it and try to make that very service even better.