The very first question in this r/games interview with Xbox Live director of programming Major Nelson elicits an encouraging answer, one that Microsoft should be trumpeting, not whispering.
After the Xbox One servers are shut down at the end of the new generation, will Xbox One games still be playable?
The question is highly relevant, because Microsoft says that the Xbox One must connect to the Internet at least once in a 24-hour period for any games to be playable on it. Imagine, a decade from now, Microsoft shutting down whatever server checks for that. Imagine then trying to play any games in an Xbox One library.
If we're taking Microsoft's 24-hour check as real policy—and they've given us only a bit of fine print to suggest that anything will ever change—then it sure seems like an Xbox One's game library would be rendered inert down the road. And that would seem like a very major reason to be leery of spending money on Xbox One games.