Yeah, but then it is yet another layer of configuration slapped on top of the previous layer of configuration. That can't be the best solution, can it? Same thing for piping helm template through Kustomize.
In my book the main problem with Helm charts is that every customization option needs to be implemented by the chart that way. There is no way for chart consumer to change anything the chart author did not allow to be changed. That leads to these overly complex and config heavy charts people publish - just to make sure everything is customizable for consumers.
I'd love something that works more like Kustomize but with other benefits of Helm charts (packaging, distribution via OCI, more straight forward value interpolation than overlays and patches, ...). So far none have ticked all my boxes.
But that is exactly what slog provides? The a unified interface that can be implemented by other logger libraries. Yes the Logger itself is not the interface, but the Handler it is backed by is.