Anything self-hosted is already fragile: it will go away when you don't continue to actively maintain it (paying for a domain, keeping a computer connected to the Internet etc.) or when you die.
You can design and mark-up content that will still be useful and readable in 100 years. You might be able to preserve the presentation logic (CSS-style) for 100 years.
You probably won't be able to preserve the interaction design for 100 years (without a dedicated effort -- that's why they bury computers along with the software in time capsules).
But I think it is optimistic to think that _most_ SasS hosts are going to archive content for 100 years. Preserving digital content is an _active_ process. It takes resources and requires deliberate effort.
Postscript: I'm trying to think of modern companies that would preserve content for 100 years, assuming they make it that for.
Facebook is the only significant current platform that I can even imagine preserving content for 100 years, but even that seems like s stretch. Historians might step in to archive it, but is there real value to Facebook to maintain and publish 50 year old comments on 2.5 billion unremarkable walls?
Twitter won't. Certainly Insta, SnapChat, WhatsApp etc. won't. Flickr probably could do it relatively easily but won't. YouTube maybe, but there's more to store. Something like GitHub maybe?
I can see a deletion heuristic that considers both account activity and repo activity being deployed within the next 10 years.
However I expect another evolution in SCM in the same timeframe.
Also maybe cvs/svn/git repo generally don't contain content worth preserving for 100 years. There are some historically significant or interesting repos, but for the most part you'll have a bunch of unremarkable (and duplicated) code that may not have run then and certainly won't run now.
100 years is a long time, but I do run 20+ years old Common Lisp libraries and expect them to work without modification; I'd be really pissed if they disappeared from the Internet because someone thought that 5 years of inactivity means something doesn't work anymore.