1. It is not possible to add optional member functions (which would be pure virtual functions) to a C++ class base class and then check at runtime if they are unimplemented in the object (at least not without implementing some way to query the object, which is slow). If you say to handle this by having typeid checks at runtime, look at the VFS and then notice that you cannot implement this typeid check in advance, since you cannot add a typeid check for a derived class that did not even exist when you compiled your code. Thus, you still need to use structs of function pointers in C++. Maybe you can use C++ classes for some cases where structs of function pointers are used, but you would giving up the ability to implement optional functions in a sane way.
2. It ignores all of the things in C that are absent from C++. In particular, C++ refuses to support C’s variably modified types and variable length arrays, which are useful language features.
3. It ignores all of the things in C++ that you likely do not want, such as exceptions and RTTI. The requirement to typecast whenever you assign a void pointer to any other pointer is also ridiculous.