> I could be paying for M process running my server executable (capped at N threads per process)?
Isn't this essentially AWS lambda or shared hosting? The trade offs will of course be their limitations, max execution times, lack of flexibility, and vendor lock in.
I found that AWS was too expensive because of their bandwidth. Its cheaper to just get a dedicated server with 5-10x the capacity that I need.
Doing what you said requires developer time which is more expensive than hardware, which is probably why AWS is so expensive. Some day that price will probably come down but for now its probably cheaper to just throw hardware at the problem.
> do I have to futz about with admining, patching, and hardening the OS
I think this is a problem [partially] addressed by configuration management. For example download some pre-made ansible configuration for a lamp server, and change just the settings you care about. Or load up someone's docker image (you'd still need to do updates)