Rubbing your finger on a touchpad/mousepad/touchscreen is a bit more difficult than simply using your hand to move the cursor quick enough to enlarge it. So I'd like for this accessibility feature to be proportionally easier to trigger when using those devices. Perhaps the speed of the shaking required could be a bit lower?
Is your touchpad rough and uncomfortable? I don't have any difficulty whatsoever getting it to trigger on my touchpad, even with a very low shaking speed. Not sure there's anything we should do here (but CCing Vlad in case he disagrees).
I can do it after a couple of tries, but an old person wouldn't be able to due to the fast movement required.