The main reasons I'd highlight:
TypeScript allows code to be self-documenting to a high degree: interfaces, arguments, data shapes are typed and hence present an explicit contract
It will enable the codebase to scale since the data flow is validated end-to-end.
Provides a significant level of confidence about code correctness in itself
Prevents people from making mistakes in API usage - these can be very hard to catch without a type system
Catches common errors that could easily slip into production otherwise
Enables confident refactoring
Obsoletes a considerable number of unit tests that would be written just to verify argument handling
Provides typing for the standard JS/Node library
Comes with superb IDE support
Honestly, it feels reckless to refuse such a good deal.
Use TS in strict mode. Otherwise, there's always an excuse to skip typing.
any in exceptional cases.