Why on Earth do we need a language that we can't use safely without "understanding of the way arguments to unprototyped functions are automatically promoted"?

Why on Earth do we need a language that we can't use safely without "understanding of the way arguments to unprototyped functions are automatically promoted"? Backwards compatibility and uses that are niche but vital. Programmers insist on using parts of languages long after they're discovered to be bad ideas, or when they're included in the language for very specific use cases that aren't relevant to 99% of programmers. Those parts of languages are catnip for "real" programmers. "You'd be foolish to use this technique, and you'd have to be stupid to need it for this application." "That's a matter of opinion, but it's fact that I'd have to be clever to make it work." Can't argue with someone who thinks like that.