Well, you still need some transpiling in order to remove the Flow type signatures. In practice, they end up being pretty similar in terms of needing a build step.

You can embed the type information in special JS comments that the checker understands: http://flowtype.org/blog/2015/02/20/Flow-Comments.html

Your existing minification step will remove the comments. But even if you don't remove the comments the code will still run.

You're absolutely right. Flow seems to be a very good alternative.

One thing I would like to add regarding the .js files, TypeScript 1.8 added a flag --allowJs which makes some sanity checks to the plain old .js files.

It sounds like --allowJs alleviates the need to create TypeScript definitions for everything? So if you have an existing project based entirely on JavaScript, you can gradually convert each file to .ts without having to do anything else?

That is true. You slowly add more black typescript to a white js project as you go along and decide the shade of grey you want.

Definitely an aim of TS to make it easier to continue using JS for those who prefer a half-way house, see: https://github.com/Microsoft/TypeScript/issues/4789