I don't know what to tell you, but you're clearly not cut out to be a software developer in either machine code, assembly or C or any other language if you don't understand something this basic.
Sum types aren't the be all end all to all issues, for example you can not representer pointer values efficiently with sum types. Even rust does not wrap up pointers with sum types. Now try to go back 37 years to C89 and ask yourself if they were going to require compilers to have stringent checks like the rust compiler does.
And, tagged unions are a thing and were a thing for a long time.
Of course it's too late to change all this today; it would have been too late even 20 years ago. But outside of f.ex. Linux kernel and some other super hardcore C libraries, a lot can be done for the world to migrate to safer constructs and away from sentinel values. And that's what languages like Rust do.
Super memory constrained environments have not been the mainstream programming work for decades and now remain limited to embedded / IoT. Not sure what the reservation against sum types is these days.
No one here is saying C is a great design, but in the context of 60 years ago, it worked out pretty well, and all the language which had additional runtime complexity (Pascal of course, but also Ada and FORTH) struggled because they didn't dial the right level of complexity