Your link to the mailing list thread describes essentially the same approach that bitcoin has for dealing with lock times. I would not normally describe this as “scriptless scripts,” which I have understood to me using the signing operation itself to achieve some goal that would otherwise be done in script. The locktime here is still enforced by special cased code of the consensus algorithm. If the number of things you want to do are fixed and enumerable then I suppose this is a valid approach. But it does not allow for general, arbitrary, future-defined constraints.

To give a concrete, simple example: show me how to do a return peg validation on scriptless scripts. If it can’t even do that, it is not a replacement for scripts.