My 2012 27" iMac tends to tilt downward if I shake the desk slightly, so normal vibration from using the work surface could cause it to drop over a period of time.
If you replace the HDD with an SSD the weight will be reduced by a pound, or so, so the tendency for it to drop will be slightly reduced.
The spring is only to provide counterbalance, not adjustment friction. There are two self locking nuts which could probably be tightened more against the collars to provide more friction. Using self locking nuts and collars this way is a common method for adjustment of rotational resistance.