The cost is also never just the scaling of servers. Scaling introduces are different class of bugs / issues:
- database connection limits
- caches
- state changes
- handling graceful shutdown
- speed of provisioning new instances
... the list goes on ...
Turns out, developers are cheaper than DevOps engineers and AWS budgets. So if a linear spike in servers necessitates a superlinear cost in operational complexity, maybe working highly performant web app backends is actually cheaper?