I just stumbled on this problem the other day when I purchased a hard pad with the intentions of switching between the Rotex and the ETS 150. Now I have to buy another pad for the ETS150. I was wondering why they would not make them the same but the lock mode makes since.
They are not the same because the ETS is a lot older than the Rotex. The predecessor of the Rotex had the same pad as the ETS but with the release of the new Rotex they changed to the Fastifx pad.
The idea of the locking mechanism being a problem for a new ETS sound like BS to me. If it has a shaft you can find a way to lock it. It seems a lot more reasonable to me that Festool doesn't want or feel the need to update the ETS because it is almost perfect as it is now. Why change a winning formula?