Thank you for your comment ! I am very happy that I am not the only one feeling this.

Indeed, I had the same turn off feeling whenever I see this package applied just everywhere and hearing the arguments made it worse...

I am fully with your interface separation, way easier in fact I intented to target this in my first solution. I find it a very natural and applicable solution.

Thank you for the clarification regarding CQRS, CQS and the package history. I guess I should have researched more on how Jimmy intented to use it or what problem does it solve.

Sadly, most of MediatR users (at least from my circle) uses this CQRS argument and apply it to MediatR.

Even Microsoft added MediatR in their Microservices and CQRS documentation:

https://docs.microsoft.com/en-us/dotnet/architecture/microservices/microservice-ddd-cqrs-patterns/microservice-application-layer-implementation-web-api

As for vertical slicing, even though MediatR solve or restructure your endpoints into handlers thus achieve vertical slicing. I find it a bit of an overkill to implement it just for that, and as you said there there's already a handful of solutions that you can do before standarizing this ;)

Glad I am not the only one with this point of view, Cheers !