It's actually really, really hard to overestimate how amazing having optional specified types is. I used to use C++, then went to Lua, and more recently to JavaScript with the occasional Python. I loved the freedom of getting away from the strict C++ type system, which mostly felt like it was holding me back.

But the thing that bothered me the most about JavaScript/Python was the fact that the tooling just doesn't have a clue, most of the time, what types things are. Misremember what has a member of what, exactly, or the spelling of a member, and you've got a bug.

TypeScript eliminates that kind of stupid bug and overall speeds up development by 2-3x. Yes, some of those bugs can be found in tests. But having to go back and fix them at all, tracking down exactly what the object structure is by hand (either in a debugger or digging through docs) versus "let the editor tell you exactly what members this class/object has, and what members its children have, so I can get it right the first time"?

And what about refactoring? TypeScript knows what everything in your code is. You decide to rename a member? Done! Just because there are twenty "init" methods, if you decide this particular call should become "init2", it will rename exactly the right members throughout your project.

Finally, the fact that it just has your back when assigning types prevents another entire class of bug.

No question; (optional!) strong types are a huge win, and as I said above, should be a required practice for any nontrivial app. You get the best of both worlds: Type safety without the manacles of having to specify everything.

Transpiling won't be obsolete until TypeScript is standard in browsers (where it could help improve JIT optimizations!), and probably not even then: We're looking at ES2017 before async/await is ready, which probably won't be in most browsers until 2018 at the earliest, and there will likely be more killer features on the horizon in the future that many people will want to use.