Hi all, I work on the TypeScript team. There's already a lot of feedback on the issue itself from users urging the authors not to make this decision, so I will hold back from adding to the noise on that issue. Every team is entitled to make the decisions that they feel are best for them, and I don't think it'd be productive to change anyone's mind in this case. Instead I'll just mention that I always welcome thoughts…
You can find many articles trying to explain which is best under which circumstances, but there are no correct answers.
I would wish for TS to deprecate one of the syntaxes (probably interfaces because they read as statements rather than expressions), and instead extend the other with the 10% functionality that would be lost.
For instance a type could be declared as open, to make it possible to reopen and extend it.