The reality is we as developers choose not to give languages 
  enough context to accurately infer the lifetime of objects.

The reality is we as developers choose not to give languages enough context to just do what I mean.

  instead of finding a way of expressing when we're done with instances

So, when are you done with an instance? How to enumerate all possible situations and devise a comprehensible encoding for them? Without requiring developers to keep a lookup table of lifetime declarations or other non-local reasoning in mind? Concurrency. Laziness. Runtime code replacement.

You're handwaving over fundamental research problems here. You don't know this is even possible in a way that doesn't completely hose the comprehensibility of a language.