Is this not a chance for github to make some money? I'm about to launch a site on github pages (simple html site, no backend) and there's a tiny chance of it going viral. If it were to reach the 100k limit for requests I'd be more than happy paying github to keep it hosted there.
I already have hosting with the likes of hostgator but they probably couldn't even handle 100k like github would.
For simple static pages (with no backend), I would highly recommend putting them on Nearly Free Speech (NFSN) [1] and using the free tier of CloudFlare in front of it. NFSN charges only for what you use, unlike hosts like hostgator and others that have a monthly payment of a few dollars, at the minimum. Excluding domain costs, it will cost you just pennies a year or a few dollars a year. You can also opt for cheaper bandwidth on the site if you wish.
Without ClouldFlare in front, your costs on NFSN may become quite high depending on the size of the content and number of requests.
I used to host some stuff on nearlyfreespeech. I liked the idea in principle, but keeping my account topped up and them charging me extra fees just to pay them became annoying. I switched to Amazon S3 and couldn't be happier. It was harder to setup but worth it.
To add to the list of alternatives, try netlify it has a free tier with no build limits and provides a pretty liberal and free open-source plan that has more feature than GitHub pages.
https://netlify.com/open-source. Just throwing out there as well.
I already have hosting with the likes of hostgator but they probably couldn't even handle 100k like github would.