There are many types -- it is phrased generally and may lead readers to believe that there is some issue with polymorphism. I still object on the subtype groun

There are many types -- it is phrased generally and may lead readers to believe that there is some issue with polymorphism. I still object on the subtype grounds though -- because the issue is not the polymorphism (that is useful and usually a good idea). It's the inheritance relation, which is not the subtype relation -- eg., you can do subtype polymorphism with just implementing interfaces and there shouldn't be any problems. This comment is actually expressing issues which have arisen because of inheritance and confusing it with polymorphism.