Sorry, if there are N clients, and M servers, then there are N+M endpoints, but N*M links, which is a lot more.

The link is only as strong as it's weakest... link? Anyway, given the structure of internet, I wouldn't say it is O(N*M), more like O(the number of major ISPs).

Well all endpoints might differ but the links might rely on the exact same algorithm so you need to count distinct links

Most internet traffic is cosolidated through a small number of providers like Cloudflare and AWS.

Cloudflare being, if it wants to be, an epic MITM given its control of DNS and its role as WAF. Line level surveillance barely matters now.

Except for countries who hist companies who hold all the endpoints.