I think the answer is that it's a lot more than you think, but I posit the problem is discoverability. You just can't go on Google to find it anymore.
It may sound like a pedantic point, but the truth is that there are more websites than before and so even if you can't know about it because Google Search really sucks nowadays, maybe one day they could be found either through luck or a better search engine taking shape. Point is, discoverability is different from not existing at all.
As for sites that eventually become defunct, we have archiving tools that somewhat work. The Internet Archive could be far far better:
- stop bowing down to takedown requests
- make the archive and its tools easily self-hostable
- make it so a P2P network can distribute it
- improve the JavaScript response logic recording as if you really were on the site
