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I think the point is "for the level of abstraction it targets". Add all those abstractions that are generally a good thing in a perfect world, and practically you shifted the language enough to be unsuited for many tasks we use it today.

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Polymorphism doesn't have to add runtime overhead. You can achieve polymorphism without RTTI through code duplication, as C++ templates do. (You can also use uniform value representations, like C void *, although this is limited in its applicability.)

Code duplication is an overhead. Not as bad as RTTI, but static memory footprint is still important in the embedded/systems world -- sometimes more so than runtime memory footprint!

You don't need any RTTI or code duplication for polymorphism, so long as you don't need general support for type-checked downcasts. If you do need type-checked downcasts, it can be as cheap as a single pointer on antirez's mylist->ops function table - a pointer pointing to the ancestor function table.

(Specific support for any single downcast is trivial - just write a function for the function table, typed appropriately and implemented appropriately.)

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