I thought I was building a C replacement. I was wrong

I agree, that sometimes doing things OOP-style with methods isn't the right choice and using plain structs and free functions is a better way of doing things. But there are cases where using methods is inevitable - when internals must be hidden in order to prevent incorrect usage. Also it's needed if one needs runtime polymorphism, in such cases (in C++) inheritance and virtual methods come to rescue. Doing so in C way (with void pointers and structs containing function pointers) is less ergonomic and bug-prone.