I would say for me, typechecking part of ts is less useful than the information it provides me about the project. I can jump in a new project and quickly see what kind of data structures are there, what are the parameters, get really nice intellisense and documentation.

All of this is possible with jsdocs and flow but they are not nearly as powerful as ts and often times verbose. I don't like polluting the comments.

You don't need to use typescript everywhere although the popular zeitgeist will tell you to avoid any or ts-ignore. It's fine to write some parts in ts and others in js. Think of ts as eslint, jsdocs or flow. You don't need to be religious about it. Take its benefits wherever you can or do you think all the other tools are useless as well?