But what is "legacy" or not is not to do with technology, but fashion. Let me give you an example: everyone rants on here about what awful languages PHP and JS are. Is COBOL really a worse language than either of those? If so why? What algorithms or data structures can't be implemented in it? What tooling doesn't exist?

The only reason COBOL is considered a legacy language is because it's unfashionable, and a large part of that - I'm not even kidding - is the clothes COBOL programmers used to wear have fallen out of fashion. There's actually no reason that you couldn't use it for any application you might want to write today, and it would probably be more productive to do so than some modern languages...