YAML is okay for writing structured prose for humans. It’s terrible for anything consumed by programs because even that single microservice has a high likelihood of some problem caused by YAML’s magic typing, silent data loss due to indentation, etc. unless you pair it with a separate validation tool chain, making the argument for simplicity increasingly dubious.
Validation is required, yes.
Sure, so at that point how much are we really saving versus using a better alternative? Using YAML correctly is harder because you need not only to do the validation everything needs to do but also doing other things specific to YAML to avoid problems created by YAML rather than the problem domain. For example, if typing less is my goal isn’t it easier to, say, always quote country_name rather than have to run a separate validator which catches the Norway problem?
Why not pick a config language that works with our current config formats, looks like out current config files, and addresses many of the dumb problems that arise only in current config choices?