Take TATV out of the equation, which I'm inclined to do (because it feels like it was written in isolation and utterly disconnected from the other 19 or so episodes before it) and there's no closure.
I don't need closure since I know what happens in the 24 seasons of Star Trek that follow it.
Same here.
And what exactly is a "proper finale" anyway?? TOS ended without one in 1969 after all.