We don't offer .io domains. Nor do we offer any of the TLD's that seem to be popular with some startups. Even though we could make money selling them. Like our competitors do.
Although popular (with startups) they aren't mainstream with the public. If you become successful you will just end up buying (at a much higher price) the .com equivalent. Or end up having misdirected mail and/or users or investors or bloggers or reporters etc. Whatever .com you can buy now will be much more expensive once the .com owner sees you have funding and a business model and a website.
I've been doing domains since the mid 90's. My strong advice is to stay away from anything but .com for your startup.
Despite what you may think there are plenty of .com's you can register (try leandomainsearch.com for ideas (I have no affiliation to that at all but use it from time to time).
And not all people holding domain names are vultures, cybersquatters whatever you want to call them. Some are actually fairly reasonable (I do some consulting on the side where I help people buy domain names).
[1] No we are not looking to get HN business if we were I'd have who we are in my sig.
[2] We aren't cheap.