It isn't just a matter of tooling. There exist hard limits on how much can be inferred about unannotated programs, and when you go past those limits, the price

It isn't just a matter of tooling. There exist hard limits on how much can be inferred about unannotated programs, and when you go past those limits, the price you have to pay is to embed (partial) proofs of correctness in your own code. For example, think about why GADTs don't play nicely with type inference. IMO, machine assistance is useful to the extent it relieves us humans from work. In particular, types are useful to the extent they can be inferred. Beyond that, you still need to prove the correctness of your programs on your own, so there is no point to the ceremony of writing down those proofs in a machine-checkable format.