Formal methods are cool because, by contrast to tools like the borrow checker, you can prove some very "nonlocal" properties: this system does not deadlock, or it makes progress at least every N steps, etc.
We can't know for every possible program if it halts or not, but the complexity of programs we can determine is increasing as tools and techniques get better
TLA+ is generally used to specify a "toy model" of some complex distributed system. It's not intended for end-to-end proof, for that you'd just use Coq/Rocq or Lean itself. Lean is certainly expressive enough, but you'll have to translate the time and non-determinism modalities of TLA+ as part of the Lean development.
