Earlier quoted context omitted.
We used to have player run servers for years. Is it some lost skill to write software that way?
It's not a lost skill. Spinning up a binary and replicating actors across two computers that both have a connection string to a server is.. for all intents and purposes: easy. Where it falls down is when you start to have complex interactions with AI that's serverside, or you have a dynamic world that changes based on player behaviour, or you have cross platform requirements, integrations with companion apps and abov…
The response of "but that isn't any fun!" is totally irrelevant; you can't preserve the initial experience, but you can preserve the basic software itself so that players still have something to mess with.
Programming-wise, this requires a little more emphasis on a modular implementation that needs to be considered from the start. Otherwise, it seems pretty straightforward. Or am I missing something?