Sorry, I didn't qualify "always feasible" well enough. What I meant was, I want it to be feasible to reason about the halting states of every single smart-contract that will ever interact with my code. Not just the ones that I write, but also the ones that are reachable from my smart contract's call graph, and the ones that call into my smart contract.

If I can do this, then I can feasibly reason about how my code will react to other peoples' inputs (e.g. I can prove the absence of DAO-like re-entrance bugs). The fact that Ethereum allows smart contracts to be written in undecidable languages means that this is not feasible in practice.