I don't get the 'maintaining type definitions is hard' argument. It's super easy to do, especially when coupled with typewriter which will automatically generate types for your server side DTOs. Also having typings saves you an incredible amount of time with fast refactoring, auto-complete, and of course the type checking which saves countless hours of debugging the dumbest mistakes.

When you start talking about getting into code bases that you didn't personally write. The time savings and productivity benefits that TypeScript brings are immense.