It was a simplified example.

B and C can’t share any other attributes without duplication of code

struct A {
    [[nodiscard]] virtual std::uint32_t DoSomething() = 0;
};

struct B : A {
    std::uint32_t DoSomething() override { 
        return 0;
    }
};

int main() {
    B b;
    b.DoSomething(); //no warning
}

even if you use virtual attributes aren't inherited.

If you ment members instead of attributes this is unrelated to virtual:

struct A {
    std::uint32_t a;
};


struct B : A {
    void DoSomething() { 
        std::println("B"); 
    }
};

struct C : A {
    void DoSomething() { 
        std::println("C"); 
    }
};

Now the member is shared and it works fine and there is no virtual.

There’s also no strict enforcement of inheritance.

True, but neither is there when just using virtual:

struct A {
    virtual void DoSomething() = 0;
};

struct B : A {

};

This compiles fine if final is not added.

Not that its an issue as reflection has solved this problem:

consteval std::string_view HasMethods(std::meta::info impl, std::meta::info iface) {
    constexpr auto ctx = std::meta::access_context::unchecked();
    for (auto fn : members_of(iface, ctx)) {
        if (!is_function(fn) || is_special_member_function(fn)) continue;
        bool found = false;
        for (auto cand : members_of(impl, ctx))
            if (is_function(cand) && has_identifier(cand) && identifier_of(cand) == identifier_of(fn) && type_of(cand) == type_of(fn))
                found = true;
        if (!found)
            return std::define_static_string(std::string(identifier_of(impl)) + " is missing " + display_string_of(fn));
    }
    return {};
}


struct A {
    void DoSomething();
};

struct B  {

};
static_assert(HasMethods(^^B, ^^A).empty(), HasMethods(^^B, ^^A));

// g++ -std=c++26 -freflection main.cpp
// main.cpp:30:41: error: static assertion failed: B is missing void A::DoSomething()
// 30 | static_assert(HasMethods(^^B, ^^A).empty(), HasMethods(^^B, ^^A));

Now you get the strict enforcement of inheritance and no virtual.