But wouldn't something like LanCache work for ISPs too to cache the main game CDNs?
Yes, it works by pointing the CDN names to your cache. Those servers are cleartext.
https://github.com/uklans/cache-domains
If someone uses a non-ISP DNS server it is their problem, most home users won't bother doing that.
I saw an unrelated example of setting the DNS server address in a PS4 to download much faster from somewhere else.