Obviously ADTs are probably just a standin, but I think they are a feature absolutely every language ever should have. They are also not very expensive, esp. runtime wise they're (afaik) equivalent to the clunkier solutions (like classic enums).

Have you never had a thing that could be EXACTLY one of two things, ever? And you wanted the compiler to make sure that, anywhere you used that thing, you had to take care of BOTH of those possibilities? That's one of the main uses of ADTs.