As a long time TypeScript user, I do think this is a design mistake with the language. For example, it breaks instanceof checks.

However:

- The language primarily uses structural typing. If two classes are compatible, it's not completely unreasonable to expect them to also behave in a structural way (though I would prefer them not to).

- Adding a private field to a class makes it work as a nominal type (https://michalzalecki.com/nominal-typing-in-typescript/#appr...). It feels a bit hacky and it's not very well documented, but you could argue that until a class has some private state you can't publicly access, there's no reason to prevent you from declaring a compatible class.

- From my experience idiomatic TypeScript rarely uses classes, the primary exception being React components. In teams I've been a part of we've always written TypeScript (and JavaScript) like an (impure) functional language, taking full advantage of ADTs, simple structurally typed data and more-or-less pure functions.