I suspect the content on Medium is now sufficiently valued that if Medium doesn't stay around, someone will make sure the content is copied and served somewhere else.
Assuming that Github static pages infrastructure is maintained without breaking CNAMEs for the next half century, and assuming you pay for your domain 50 years in advance, maybe the static website can remain for that long?
Only if your site gets picked up by archive.org though.
(That's only going to happen with static sites, not those which require login or signup, etc.)
Even then your ideas will still effectively disappear because search engines don't appear to crawl archive.org. (If they do it's incomplete - my own past content on archive.org does not get found by specific Google searches.) But perhaps search will change in future.
When you get sick, poor or die, your own platform will almost certainly disappear, taking your content and ideas with it.
If it's on Medium, your content is more likely to survive your demise.