Looks like you answered your question yourself.
Of all the things the dummy teaches the hands are the least important. When people go fast what do they focus on? Their hands!
One of the things that makes some wing chun teachers say"Why bother".
sifu says its about sticking to the dummy,not just making a lot of noise,and that most sifu's actually can hear when their students are doing the form correctly or not!
...I simply was referencing that the play/rebound from such a wall mount must have been absent in the mook work before Yip (Ip) man moved to HK and started using his wall mount...
I absolutely do see the improvement with the use of a LIVE dummy... I guess I wanted to also assert that though the LIVE mook is a great boon to training WC, WC survived quite well w/o the LIVE mook for more than a generation or two.