I see this argument all the time but its not the fork() that is the most expensive anymore, its the actual program initialization after the fork(). If your app is non trivial (lets say a websocket based chat server that needs to persist messages to a DB and use pubsub to sync them to other processes) it probably needs a connection to a database, a connection to a cache server like Redis or Memcached, etc, or perhaps a connection to some other backend service. Reinitializing these dependency connections from scratch in a new forked process for every single incoming websocket connection is expensive and slow.

On the other hand a Node.js or Go program can have a preestablished pool of keep alive connections to the backends already ready to go and reuse that connection pool for many hundreds or even thousands of concurrent websocket connections. You can approximate something like this with the fork() model by having a local daemon process that manages the connection pool and have your forked process talk to that local helper daemon when it needs a connection, but you are still going to pay a penalty for that compared with having a fully preestablished connection ready to use right there in the process already