I agree with you on MustOverride.
It's like creating an interface or an abstract class in Java, the derived class must override the method.

I'd like to stree that the inheritance contributes the power of the object oriented programming.
Therefore, all the method must be overridable. We do not have to say it. If we cannot override method freely, what's the purpose of using the OO language.
That's why, the keyword overridable and overrides are redundant to me and the fact that we cannot override methods without them defeats the purpose of the OO programming in my mind.