This is just one reason why we need interface inheritance

gistfile1.cs

IList<string> list = new [] { "I", "declare", "you", "are", "mistaken", "sir" };
IEnumerable<string> youProposeIShouldNotBeAbleToDoThis = list;
// For the above code to work, IList<T> must inherit from IEnumerable<T>, i.e. interface inheritance must exist.

So really you're for Interface segregation. I agree. In this case it seems like Command/Query separation. Happens a lot.

I think I would understand better if you present A) this is how it was B) this is how it should be

Yeah that's pretty much it.