There might be cases where you wanted to quote someone else on old posts but you can’t because the thread itself is locked. Doing this manually actually take some time, at least for me. This is probably because the thread itself is locked so replying isn’t possible (the tab where you will type won’t appear).
Also on the other hand, isn’t it possible to lock all old threads manually? Although 1.5k+ is just too many and might take a whole day to do it.