The problem is that almost every developer who comes to serverless is very aware of how to build things with servers, instances and containers.
Having run tech teams, this principle is simply to stop them going back to “what they know” as a default position.
In that sense, it is intentionally harsh because without it, bad habits persist in the majority of developers.
The long term cost of running instances, servers and containers are also so much higher than most developers are aware of. Getting them out of that mindset is absolutely vital.
There are very good reasons to use these technologies, but they should be business reasons, not technology reasons, such as consistent, predictable, workload. I’ve very rarely come across a scenario where that was a valid reason for not using well optimised on-demand compute for example.