And it is quite obvious to me that you need to know BOTH TypeScript and Javascript, of course, you cannot just learn TypeScript, because it is just a thin layer on top of Javascript without many runtime guarantees otherwise.
You seem to be conflating objective wrong/right with your own personal stance. Do you have an objective argument in favor of your assertion about the longevity of non-TS libs?
> And it is quite obvious to me that you need to know BOTH TypeScript and Javascript
That's a reasonable strategy on the assumption that TypeScript will be used, but the very fact of its being so is actually an argument in favor of the point made by gp which has been left unaddressed (RE context-switching).
There is no need to context-switch. Just accept Javascript as part of TypeScript, because it really is. The argument is clear: TypeScript drastically reduces the amount of errors you will be making when coding, and the amount of time you need to think about stuff that is really trivial.
The rule of thumb on how to do this is also simple: Keep your types simple, use them to make your life easier, not more complicated. If you cannot model something simply using types, don't try to do so, just use the dynamic typing escape hatch that Javascript provides.