at a certain scale you have to assume that at any given time at least one of your users is guaranteed to be doing something dumb that will cause performance iss

at a certain scale you have to assume that at any given time at least one of your users is guaranteed to be doing something dumb that will cause performance issues. if you haven't implemented rate limiting or request prioritization at every layer of your system in a fully shared environment this will break everything for everyone instead of just breaking it for the one bad actor. having fewer shared components means having fewer ways for this to go wrong. even multiple app instances and multiple db instances on the same machine isn't necessarily enough if you don't do proper isolation with containers or vms due to noisy neighbor issues. and don't even think about sending requests from different users to the same hdd because there is almost no way to isolate them appropriately, ssds make this so much easier.