But what about when there will be problems with your single point of failure? Sometimes you'll run into situations where an instance of any software has degrad

But what about when there will be problems with your single point of failure? Sometimes you'll run into situations where an instance of any software has degraded performance for reasons beyond either your control or understanding. That's one of the situations where horizontal scaling let's you deal with such circumstances better, instead of outright impacting everything and everyone connected to it. A front end web server container crumbles under the load? No worries, just redirect the traffic to others. A back end API container has problems with the JVM reserved memory and/or the thread pool for processing connections has been filled and all new incoming requests just get queued up? Just have more instances to cope with the load and don't let one instance under load affect all others. But what are you supposed to do if long running SQL exhausts the resources of your DBMS? What if a background process needs to run some really complicated writes, but your application needs write capability without slowing down? That's not to say that bad configurations or bad code shouldn't be addressed, but any means of fighting it and minimizing the impact is worthwhile in my eyes.