I understand the warning in general. But isn't .io, specifically, connected with the British government? And isn't the British government fairly stable/pro-business?
The resulting arrangement would virtually certainly be something like Tuvalu and .tv. Nobody is suggesting that they go into a Freenom-style domain mess like .ml and a few other domains (which some countries decided to just give away for free).
But if you choose .io, you're gambling on the future of your internet property in a way that the owners of traditional domains like .com simply don't have to worry about.
What if someone decides to start a media campaign for divestiture of .io domains? It's not particularly likely, but it's a risk to weigh against the benefits of a cool domain.