Horizontal scaling works!
But, it’s only efficient in convenient, shared-nothing type architectures. If you are dealing with longer lived, more stateful stuff (like a game server perhaps) it’s not going to be so easy.
Of course, it’s easy for fancy VC funded stuff to defer optimizations later. If you are working solo you are probably going to want to, counter intuitively maybe, spend more time thinking about how to keep things optimal and simple. There’s probably startups out there worth real money whose entire stack could run on a $20/mo DigitalOcean box if it were better optimized, but actually runs on a $1000/mo Amazon or GCP account. Of course there is a lot to be said about that (obviously, the latter set up should, if done well, be a lot more resilient and prepared to deal with scale) but it does go a long way to demonstrate why someone might care about efficient async; the scale of money you can burn when you are bootstrapping or doing solo hobby projects is a lot different from well-funded startup or established business...
Everything has tradeoffs. I do believe Go wins in context switching, but the GC latency and CPU usage has definitely caused some problems. The Go developers have done an impressive job with GC latency and performance, but it’s hard to be cheaper than free, so sometimes Rust is a better option, for example.