Yes, you are of course right...10 years + (however many years the legal process of transferring sovereignty takes) + (some number of extra years because stuff happens) from now I would completely agree it probably won't be around. But from an internet perspective for a mostly fad driven TLD I personally would expect on the day .io finally disappears from the nameservers the reaction will be "oh...that was still around?" or worse case a tiny number of "I forgot we registered that".

Tl;dr: "someone will likely run .io until ICANN turns it off, which it probably will, but we don't know who that is right now".