> Everything on the same LAN should generally be treated as "compromised/not compromised" together. There's rarely just a compromise of one machine in the same way there's never just one cockroach.

I would not generalise so quickly. Is every computer compromised in the internet if one is compromised?

No. It highly depends on the trust between those machines and whether they share similar services with critical vulnerabilities. Only then, they might be compromised together.

But the world has evolved and not everyone anymore bases their total trust and security thinking for "no outside internet connection, we are fine".