In Chapter 8 of MES, Rothbard cites Bohm-Bawerk's claim that there's always a more roundabout process which is more productive than shorter processes in any line of production.
To state it most simply: For any process for producing units of X, there is some other process with takes longer but is more productive of X. This says nothing directly of the quantities of inputs, or even that the factors are used. Obviously, that's not to say that all longer processes are more productive than all shorter ones. That would lead immediately to absurdity. The assertion then is that there is at least one longer process more productive than any of the shorter ones.
If the longer process uses more of all the same inputs as the shorter one, then what of a process as short as the short one but which uses the same increased quantities of inputs permitted for the longer one? Wouldn't B-B's assertion say that the longer one must still be more productive than either of the short processes?
If the longer process uses different factors than the shorter one, then what of a process as short as the short one but which uses the same quantities of the same factors used in the longer one? Wouldn't B-B's assertion say that the longer one must still be more productive than either of the short processes?
If B-B's assertion does hold for all pairs (P,p), where P is the most productive process that can be done in t+1 and p is a process taking all the same quantities of inputs as P but which is complete in only t, then doesn't B-B's assertion lead to the conclusion that for any particular set of inputs, the yield can be increased indefinitely by increasing indefinitely the time permitted for the period of production and pursuing the most productive process that can be completed in that period? Does this violate the Law of returns?