You are right in everything you say, and I'm just replying to your comment because it seems like a good place to say my piece:

Why do we (devs) know that one-letter variables are bad, but still don't know that one-letter generic types are bad?

Why do we put up with this (random function from rxjs):

    export function concatMap<T, R, O extends ObservableInput<any>>(
      project: (value: T, index: number) => O,
      resultSelector?: (outerValue: T, innerValue: ObservedValueOf<O>, outerIndex: number, innerIndex: number) => R
    ): OperatorFunction<T, ObservedValueOf<O> | R> {
      return isFunction(resultSelector) ? mergeMap(project, resultSelector, 1) : mergeMap(project, 1);
    }

Why T and not INPUT_TYPE or whatever it is (see? I can't tell!)?