If you ever want function types you will always need both co- and contravariance as pointed out in OP's articles. Using covariance as a default is also not sou

If you ever want function types you will always need both co- and contravariance as pointed out in OP's articles. Using covariance as a default is also not sound. Many things are "invariant", meaning neither co- or contravariant. This is already a sensible default. (It is however possible to automatically derive either property in some cases)