I don't think many real servers have 4 GB these days. The dev box under my desk has 32. Linux servers often have 1 TB of physical memory. So based on your calculation that's 256 million threads. Sounds enough :)
The whole point of a cheap concurrency like goroutines is the ability to avoid using callback based async IO (i.e. the go standard libraries perform select/epoll/... under the hood but your code behaves like if it was blocking on IO).
You might have better use of your memory (caching, in memory databases) than wasting it for even a fraction of a million of routines that are just waiting for something to happen.
Yes, but if I buy a server with e.g. 128 GB of RAM, I would preferably spend it to cache 100GB of live data and searchable indices, and not worry when I will spawn too many threads so that I will start trashing the swap.
You might have better use of your memory (caching, in memory databases) than wasting it for even a fraction of a million of routines that are just waiting for something to happen.