Also note that the OP said “avoid extending classes”, but didn’t say “avoid implementing interfaces”, so they don’t disallow inheritance in a wide sense of the word.
I think “avoid extending classes” is there because it is as good as impossible to design classes that can be extended easily in ways you do not foresee, and if you do foresee how your classes could be extended, it often is easier for your users if you made your classes more flexible, to start with.
You can get Encapsulation, abstraction, and polymorphism any number of other ways. Inheritance is the only defining property of OOP.
If you removed all the stuff related to inheritance and trying to fix the leaky abstraction that is objects, the language would be a fraction of the size (compare with Go or StandardML for how small a language without inheritance can be).
