When your service can no longer process requests as fast as they come in, you've hit a wall. Until then the simple solution is to allocate more resources to your service (i.e. scale vertically).
There's obviously a balance to be struck but the more often you do something the easier it is to do. If you have a simple app with a db, backend, frontend and proxy, vertically scaling every time you hit that wall is going to be very painful. A little complexity goes a long way - using a managed system adds a very small amount of complexity in exchange for some breathing room when you need it. The last thing you want when your service is in a death spiral is to start thinking about the practicality of migrating your db and taking the hours of downtime/working overnight/weekends to do it at a convenient time for your customers.
